Id say if you are only plugging into one Cell of a 3 cell, you could be discharging one cell slightly more than the others, which is bad for batteries, but it depend on the final voltage of the batteries after youve discharged them. If you were only using the camera, you could be discharging one cell so much you cause it to be more worn out than the others, but if you are discharging the other two cells as well you should be ok over the course of the lifetime of the battery, as long as you are relying on a balance charger to get them back to a balanced charge every time you recharge.
the VTX i always buy has input voltage of 7-36V, and a output of 5v for the camera, so i just splice directly into the ESC leads. When i plug in the battery, i plug in the VTX too, directly to the full voltage of the battery across all cells, with one connection, seems to work for me. if your VTX is rated for at least 12v, you could probably get away with it under full voltage of a 3S Lipo(12.6-9.0V), it really depends on what you are using, a 3.3V tiny whoop VTX wont like that at all.
this is sorta a non answer, but in 2020 with modern charging equipment charging modern batteries, you should be OK, to the point of never noticing a difference, but you might get a few more charge cycles (like 250 cycles vs 225)of charging and discharging if you baby your batteries a bit more. That being said, a few hundred miliamps wont make that much of a difference.