Hi!
I have the Canon SL1 and am upgrading that...I think!
My go-to lens is the EF 24 MM F2.8 IS USM which fits both the Rebels (but at the crop factor of 1.6) and full-frame.
I found a significant improvement in image quality going from the kit 18-55 EF-S 3.5-5.6 IS lens to this lens. Just something to keep in mind, that is, the lens you use and its impact on photo quality. You might want to demo a good lens and see what a difference that makes. If you like image stabilization be sure the lens has it though. Maybe you don't need to upgrade the SL3, just the lens. Just a thought - don't want to steer you away from what your heart is saying.
I would ask why the T3 is not working for you and why want to upgrade. I have been using my SL1 for over 4 years now and when I get frustrated with it, I realize I have something to learn and it is likely not a fault of the camera.
Of course, now I am thinking in-camera focus stacking, image stabilization of body, weather sealing, a higher dynamic range... so new features!
Do you have a camera store nearby?
You can demo lens and camera usually right there without cost. Or, at least go to a Best Buy and hold cameras in your hands and see how they feel. That is so important to me. I am small and use my camera during hikes so weight is important as is holding the camera steady to reduce camera shake (even if a camera/lens is stabilized it still helps me to get sharper images if I hold the camera in a manner to reduce/eliminate shake)
You might also want to check out the very light full-frame EOS-RP (my main consideration now). If you have EF (full frame) Canon lens you can use those on the RP or R with an adapter. It is lightweight and felt great in my hands.
That said, for APS-C I am considering - like you - the T7i or the 80D (older yet still good from what I read). I have not demo'd either though, so cannot offer feedback.
And to answer your question which others did I believe - yes! Your APS-C lens for the T3 will work on another APS-C camera.
Good luck and enjoy!
Annie
