Those look great! Personally, I'd separate them out now before they become too big to easily handle. I also planted a pot-full from seeds and then took the two biggest and up-potted them. The rest were discarded as the original fruit was a white-type and wasn't really that good. (I was more interested in experimenting with propagating from seed.) From the many seedlings I started with, I now have 2 dragon fruit plants, both about 3' tall after a year or so of growth. Both of these are in relatively small pots and have been kept indoors the entire time. As a result, they are a) probably root bound and b) a bit spindly from not enough light, but as I mentioned, I'm not really interested in the fruit from these two as I have a couple of others. They are starting to get too big to keep them indoors so I'll have to do something for them.

As soon as they grow too tall to stand up straight on their own, I'd stake them. They should be put in their permanent home before they get too big to easily move. Each should be in it's own pot (at least 2-3 gal, preferably a bit bigger) or in the ground. I have one in the ground and one in a pot resting on bare dirt as it had gotten too big to easily transplant. Of course, that is the one doing the best. Both share the same climbing post and one has produced two flowers.

They are relatively fragile and I've lost 18" of growth when I mishandled one (the still-potted one) and the top snapped off. These plants can get BIG! So you will need a beefy trellis to support them. Lots of examples out on the web.