The Sora 2x derailleur mech that came with my gravel bike broke in half. Is the 2x Tiagra/105 front derailleur compatible with the R3000 shifters? Thanks!
You can get the Shimano specs and compatibility info here (expand Technical Documents on left hand side).
The front drivetrain compatibility chart says the ST-R3000 lever is only compatible with the FD-R3000 front derailleur.
If you study the compatibility chart you'll see there tends to be a lot of compatibility between drivetrains that match in number of speeds, but not between speeds. For instance 2x10 Ultegra 6700, 105 5700 and Tiagra
4700/4600 are all compatible with each other but not 2x9 or 2x8 group components.
I recalled a question where the asker had a Trek bike with a Sora drivetrain and 105 front derailleur. Obviously Trek thinks this works and Shimano are known to be conservative in their specifications and compatibility recommendations.