Horses for courses. if you are mostly urban I find 3 better overall than O2, espically M4 corridor/thames valley. Ironic given their respective HQ locations.
Mind you the gaping vortex signal dropout just at the train end of the platforms in Paddington is amusing since you can flick the V's at Voda's HQ from there.
Its so localised I'm almost convinced there is some blocker or alternative transmitter in operation as every network seems to get hit.