What is the easiest router to flash that isn’t gonna cost an arm and leg and also be able to reach 3 stories without connection loss?
3 stories of what, wood, concrete, rock?
And is it intended to be placed on the middle floor, or top/bottom floor?
Bottom floor and concrete
Most likely none, you'll need APs 
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I agree with @diizzy. With concrete construction, you're very unlikely to be able to have good wifi through a single floor, let alone 2. If your AP is on the bottom floor, you will not get any reasonable coverage on the 3rd floor at all. At the very least, you need to place your AP in the middle floor, but I would recommend at least 2 (top and bottom floors), but preferably 3 APs (one per floor). For maximum performance, you need them all to be wired to the central router -- wireless/mesh uplinks won't work well.
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Or power lan adapters, attached to APs or being APs.
But performance will suffer.
Love my WRT32X got it for $150 on amazon, 100+ day uptime, fast and almost flawless, but wifi is hit or miss without 1-2 U6-Lite APs for a big house like it sounds like you have. Therefore I'd recommend as follows (it's old wifi 5 but works well):
-
Buy this:
https://www.amazon.com/NETGEAR-Nighthawk-Ethernet-Wireless-Optimized/dp/B07KXWG9Q2/ -
Flash 'factory':
https://firmware-selector.openwrt.org/?version=SNAPSHOT&target=ipq806x%2Fgeneric&id=netgear_xr500 -
then SSH in and install this:
opkg update && opkg install luci luci-ssl irqbalance luci-app-sqm luci-app-adblock luci-app-upnp luci-app-wireguard luci-app-samba4 kmod-usb3 kmod-ata-marvell-sata kmod-usb-storage kmod-usb-storage-uas block-mount usbutils mount-utils luci-app-hd-idle kmod-fs-ext4 kmod-fs-exfat iperf3 nano
Next version you won't have to do all that since it'll be officially supported (same hardware as R7800).