There seems to be two different hardware versions (the flash chip differs). On my RB750Gr3 18.06.1 won't boot because it can't find the flash. See my post above from October.
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If there won't be any u-boot compatible builds in the future, will anyone be willing to contribute backup files necessary for reversion to the "stock" Mikrotik layout. Yes, I am aware that without my original backup and the license therein I probably won't be able to boot RouterOS, but I got the device exclusively for OpenWRT, so that will not be an issue.
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I would also appreciate it if someone could post backup files for "stock" Mikrotik layout. Additionally, instructions from a working 18.06 installation to a snapshot version would be appreciated by probably quite a few people.
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Binary of RouterBOOT + RouterOS (i.e. original binary data of my device):
https://gist.github.com/jwangac/701a31f0cfe03ad6bef821affd005944/raw/db6bed1a6a9a898af0c61cf616ea3855baca4bd7/RouterBOOT_RouterOS.bin
Binary of RouterBOOT + OpenWrt (snapshot):
https://gist.github.com/jwangac/701a31f0cfe03ad6bef821affd005944/raw/db6bed1a6a9a898af0c61cf616ea3855baca4bd7/RouterBOOT_OpenWrt.bin
Use a flash programmer to test them. Since I no longer have this device, please use at your own risk.
Note that MikroTik have started to use a new flash chip (W25Q128JV) on newer devices, so the binary (from my older device) may not work.
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@jwang Thanks for the links. Luckily my device has the old chip, just like the images posted above. I assume that the only way to write this is with an SPI programmer. I'm going to ask around to see if anyone has one I can borrow.
If someone knows of a way to do this with a ramdisk booted from u-boot, that would be quicker and very helpful.
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Using an SPI programmer, the images linked by @jwang worked flawlessly. Thanks again for your support.
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Hello,
I have a Mikrotik RB750GR3 router and I tried to install OpenWRT like described in the Common Procedures for Mikrotik RouterBoard Products Wiki pages.
OpenWRT boots from the downloaded image (https://downloads.openwrt.org/snapshots/targets/ramips/mt7621/openwrt-ramips-mt7621-mikrotik_rb750gr3-initramfs-kernel.bin) and I can ping the router and SSH to it on IP 192.168.1.1 on LAN interface 2.
But the LuCI web interface doesn't show up. I tried different browsers on http://192.168.1.1
Can anybody help me?
Thank you.
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You have installed a snapshot image, which does not come with LuCI pre-installed. You have to install it yourself.
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@tmomas
Thank you for your fast reply. I see.
But then the next problem: there seems to be not enough space on the Mikrotik?
* verify_pkg_installable: Only have 0kb available on filesystem /overlay, pkg luci need 1
* opkg_install_cmd: Cannot install package luci.
Any suggestions or is there a manual for the Mikrotik RB750GR3? Thank you again.
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Ok, got it 
I transferred the sysupgrade-image to the /tmp folder on the Mikrotik and over the SSH I typed
sysupgrade /tmp/my_sysupgrade_image.bin
After reboot OpenWRT is installed and I could install Luci with
opkg update && opkg install luci
Thank you for your help!
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Thanks for the note, tm38! I too found that Luci was missing after going through the instructions for Mikrotik devices, which I've done a number of times without any problems. The wiki page should be updated with a note.
@jcwx, welcome to the community!
I've never had the issue you describe using a release version of the software - that issue only occurred using snapshots.
I suppose I was using snapshots. I grabbed the two images linked to on the wiki page here.
https://openwrt.org/toh/mikrotik/mikrotik_rb750gr3
For years I compiled my own, although I don't really spend that much time on OpenWRT for the most part. I have a few Mikrotik switches that I've set up with openflow in the past for SDN testing. I imagine I simply misunderstood what was being stated.
So I installed the snapshot image over the weekend on my device, and I noticed it is quite often rebooting, and two days in a row it is hanging. It is just a standard snapshot + Luci.
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Hello everybody,
I want to install openwrt on my RB750Gr3. I've read practically all this topic, but I still have some doubts. Hope you can help me. 
1- Which version should I install? (remembering that my RB750Gr3 is with the ROS)
Is this okay? rb750gr3-initramfs-kernel.bin
Or do I need to use a snapshot version as reported on the TOH page?
2- To install, can I follow the step-by-step found on this page? Or is it another process?
sorry for the questions, I've never done the process on a Mikrotik and I want to make sure I do it right! 
The snapshot image is based on this commit: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=52f2d7d2a9dc3e142bb957462a30806eb5cebaf4
This patch adds support of MikroTik RouterBOARD 750Gr3, without the need
to reflashing the bootloader.Installation through RouterBoot follows the usual MikroTik method
https://openwrt.org/toh/mikrotik/common
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Snapshot image work like a charm. I use it from the first commit, when is coming in snapshot release. Before that, i used a patch of a source code. Procedure for flashing is long but working... Follow link from "tmomas", or use my steps:
- Save RouterOS license.
- Make mikrotik boot first from PXE.
- Download snapshot release, from https://downloads.openwrt.org/snapshots/targets/ramips/mt7621/
- I use tinypixie for load initram version to check all is ok. (http://reboot.pro/files/file/303-tiny-pxe-server)
- Flash with sysupgrade -v... , or install luci and flash sysupgrade image through web browser.
Enjoy
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Thank you very much @tmomas and @muni !
Yesterday I flashed my 750Gr3 successfully.
Just one more question...
I flashed with snapshot release, like you guys said.. if I want to upgrade, for example, to the stable version (18.06.2) .. Can I do it through Luci?
Thanks !
Glad to help.
Don't flash with stable version yet, patch is only included in snapshot release.
don't worry for stable version, snapshot is newer as version.
I use one version from beginning February and one from one week ago, i have two boxes.
if you want upgrade to newer snapshot, follow upper procedures.
Naturally you can use saved config.
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Guys, can someone, please, take a screenshot of "Switch" tab in LuCI?