All three share the same sensor and have video, 600D and 60D have the tilt out rear LCD, 7D has much better AF system and micro adjust for fine tuning of lenses
600D Smallest, polycarbonate body, no weather sealing, Takes SD card
60D Medium size, polycarbonate body, some weather sealing, takes SD
7D Largest and heaviest, magnesium body, reasonably well sealed and takes CF card
Personally would buy a 550D and spend out on lenses, the image quality will be near enough equal as that also shares the same 18mp sensor and is available at a good price with 30 quid cashback from Canon as well
I’m still using a 40D with decent’ish lenses, but will admit the high ISO performance is better on these latest models