you need two rear facing mirrors, one of which must be the drivers side door mirror. The other can either be a passanger door mirror or the rear view mirror.
I've only got the two wing mirrors on my Elise. I took the rearview off when I bought it as it got in the way. It's passed all 3 MOTs so far. Also, vans tend to manage ok without one.
I remeber my Grandad only had a rearview and a drivers side mirror on his Mini! It would have cost him more for a third mirror and he never wasted money on unecessary things!