I don't scan my ewes, so when I have triplets I will take the most odd-one (sizewise) out after about 4 days. All three get topped up with colostrum (either form the ewe or frozen from my goats) for the first day, then also still with milk. I then rear the "spare" - could be smallest or biggest, I just make sure that the two left on are of equal size, and preferentially take away a male, to be castrated - on goats milk (which I have tons off). So the cost of bottle feeding comes into the equation, also time available etc etc.
But unless you scan, fostering on is not really possible, as you never know if the ewe will have twins or not.