I'd put them in pots of the same size as the one they've came from? Or what about in the ground? I don't really know so might not be the best answer..

Hard to say until you've actually seen the root system for each plant, and it might be difficult because, with three in the same pot, the roots may well be intertwined, one with another. The usual rule is to pot a plant into something big enough to leave a maximum of about 2 inches of free compost surrounding the rootball. Sounds like they should be in separate pots - I'm a bit mystified as to why they're all in one pot in the first place though, is that how you bought them?

Ti Cordylines are only really hardy in the tropics, so planting outside may be out. Houseplant suppliers often plant them several to a pot to provide a fuller foliage mass, but they will be happier in the long run in their own pots. My rule of thumb for that is that the pot should be around 1/4 of the height of the plant. If one or more has several branches, they will need a larger pot.