3, tesco, and virgin are VMNOs - virtual mobile network operators. they buy airtime in bulk (from orange, o2 and tmobile respectively) and sell it on. you use their sim, their services, but someone else's masts and technically have a lower priority access to them than the regular customers.

3 is a little different as they have a 3g network for data traffic which their phones use as a priority, and then fallback on to orange when needed.