At 3" tall they are just about right for replanting. Spindly is ok.
But if you are going to keep them in a pot .. good luck !
Ideally they should be planted in the ground that has already been prepared ( turned over and manured with some growmore added as well). They will grow dramatically in the next few weeks. They should also need a net over them secured at all edges. Wood pigeons love small brassica plants as do cabbage butterflies. When they lay their eggs you will get caterpillars which also love them. If you are very lucky you will have one left for yourself.
At the start of this month I had about 100 of these small plants and managed to give all but 10 of them away. The 10 I saved are about 12" tall and have a span of 20" .. they are still growing under frames and nets .. caulis really need plenty of space.
Good luck ... but caulis are not really what I would recommend if you want to see a good result after the efforts required ... if you want to grow veg why not get a few 10" pots, some compost and buy some Dwarf French bean seeds. Guaranteed you will be well impressed and absolutely no maintenance other than watering.