The most important thing is to be sure that all three get enough colostrum now. If your friend has anything other than three lovely plump bellies, he should top up the not-plump lamb(s) with colostrum or colostrum replacer. Once he's given colostrum according to the manufacturer's instructions, or for at least two feeds if using actual colostrum, then he should switch to topping up with milk.
It can be delightful to assist a ewe to rear three, depending on your setup. The lamb(s) you are supplementing soon learn to come full pelt, shouting their heads off, when they see you approach (or you call them) with a bottle.
The scenario to watch is if none of the lambs seem to want topped up, and/or the ewe prevents them from coming for a bottle feed. If the ewe is in tip-top condition, and your grass ditto, and a warm spring/summer, then with some additional cake etc, she should be able to rear three. Otherwise, if you can't get to top up any of them, I would take one off or, if the ewe seems thin, or is a first-timer, and/or the grass situation is woeful, maybe even take two off.
As to how often and how much to top up - it depends on how much milk the mother does have and how hungry the topee-uppee is. You can often get away with two topups a day, as the lamb gets enough dribbles in between times to keep going. But that can be a dangerous road to take, as those dribbles may just be the dribbles that put the ewe over the edge...
Unless your friend lives somewhere that has escaped the cold wet summer most of us had last year, the wet winter that followed and the unseasonably cold and snowy weather we've had in March, then I have to say that this year I wouldn't even contemplate leaving three on a ewe, even with topups. I'd make sure that all had plenty of colostrum (replacer if necessary), if possible leave them with mum for 24-48 hours, and then remove one or two according to the ewe's condition.