It depends on the ewe and feeding. If she is in good condition, not thin, and has plenty of milk then she may be able to manage three. You'll need to feed her extra to help her make the extra milk. Keep a close eye on the lambs and if you think any of them doesn't have a full tummy and/or is blaring, you may need to top that one up from a bottle or even take it off and bottle-rear it.
If you aren't sure she has enough condition, and/or you won't be able to cake her, then it is probably best to bottle-rear one lamb. As the lambs grow they demand more milk, and if their need oustrips her supply then you can end up with her losing too much condition and even getting mastitis, and one or more lambs foundering. They're harder to get onto a bottle later on, so if you don't think she'll manage, take one off earlier rather than later.
Leave them all on for 48 hours to make sure they get some of mum's colostrum - but if you think there's any chance they haven't all had a good bellyful of colostrum then get a colostrum replacement feed into them as well. Then if you are going to take one off, take it off after that.