I recently unearthed my EOS 650 film camera purchased circa 1990, along with 3 EF lenses (50mm, 35-105mm, 100-300mm), all in perfect condition. Would those older lenses be compatible with a new EOS Rebel SL1 body?
Yes as long as they are genuine Canon lenses those EF lenses are compatible with all EOS cameras.
Any likely autofocus issues?
Due to manufacturing tollerances there is always the possibility of slight front or back focusing of any lens camera combination.
Front and back focusing can be more noticiable on digital cameras since when you are viewing a photo on your computer screen at 100% it is the equivelent to looking at a poster size print from 18 inches away. These focus variations probably wouldn't be noticable in an 8X10 print, or even if you are viewing the entire photo on a digital display.
Other than that the lenses should perform just as well on the SL1 as they did on the EOS 650.
The key word here is "EF".
"... with 3 EF lenses (50mm, 35-105mm, 100-300mm), ..."
As long as you have the EF in the naming convention, they will work. As pointed out, newer EF lenses may be better but the older ones will still do and be as good as they ever were.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.