A:AnswerYes, it is compatible, as they are both EF mounts. The only thing to remember is that the SL3 is a crop sensor camera, so the 50mm will be more like an 80mm lens. But I use it on my crop sensor (I have an SL1) and I love it.
A:AnswerFrom a quick Google search it looks like that camera doesn't have interchangeable lenses. So no lens can be bought for that camera as it is built in.
A:AnswerI have owned 3 cannon cameras and all of them I can interchange the lens, including this lens. I don’t have a 40D to answer you for certain but I think it will.
A:AnswerYes, this lens is compatible with the 70D. I have the T7i and like the 70D it has the APS-C sensor. This lens is compatible with all canon cameras that have the APS-C sensor.