(Although overlooking the possible improvements in the main 19.07 OpenWrt since the 19.07.3 release makes no sense...)
- Checkout the 19.07 branch, not the fixed 19.07.3.
- git checkout the commit before or after 19.07.3 release tag (to get the equivalent of 19.07.3 without checking out the tag and locking the feeds). You want commit 85e04e9f46 or c3e3802a8e
- update feeds normally with "scripts/feeds update" to the HEAD of them, instead of the static 19.07.3
That should produce you the "release 19.07.3 main source" plus "package updates in 19.07 since then".
Relevant git history
Basically the release just temporarily changes the version string, fixes feeds, and that is then reverted immediately after the release and the branch continues again its normal life.
Normalization after release: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=c3e3802a8effd099481cedf669e3a17840612514
But that (checking out the release) naturally overlooks the possible package updates in the main OpenWrt source base, as only the feeds are updated (most packages, LuCI, etc.).
Note that naturally only pulls the updates from the 19.07 packages feeds, not from master. Master and 19.07 have separate packages feed branches.
So far there haven't been so many changes since 19.07.3, so you are not hurting yourself too much (unless you have a device that has got fixes since then (mainly bcm63xx, ath79))
Much more sensible would be to checkout the HEAD of 19.07 branch and build the quite up-to-date 19.07 build, instead of trying to half-mimic 19.07.3
There is nothing holy about the .point maintenance releases, so it usually pays off to build from the stable branch HEAD.