I would have done the same as you and kept the 3 with their mother and topped up. It's a lot easier to spend a few weeks topping up a willing lamb (maybe just once day) with milk than trying to force a murderous ewe, several times a day, to feed it.
You can of course wait till they are on concentrates before you give them any extra, and I've done that with later lambs when there is plenty of clean spring grass. But unless you're incredibly lucky a ewe cannot adequately feed 3 lambs.
These are early lambs and if grown to optimum potential should fetch a premium price. But unless you top them up with milk you will lose the most efficient growth phase of the first few weeks and end up with mediocre lambs that have missed the early market.