Straight from the highway code

give priority to traffic approaching from your right, unless directed otherwise by signs, road markings or traffic lights.

Basically anyone approaching from your right and that might be straight ahead on a 3 road roundabout has priority, however you have to make a judgement call regards have you time to clear the way before someone approaching from the right reaches you. Essentially you should not enter if a vehicle or other means of transport has to brake in order to let you through.

Richard