Canon model numbers make no real sense, as you said the T3 < T3i and the T5 < T5i but the T5 is their newest camera.
The 5D xxx is not replaced by the 6D, but the 50D < 60D < 70D in secession is replacing one another. Same as the T3i, T4i, T5i. Go figure.
The numbers make sense in most of the rest of the world
The T3 and T5 are 1100D and 1200D respectively, the t3i-t5i are 600D-700D
The only one that doesn't quite fit in is the tiny one, the 100D.






