They seem pretty big pots already. Are they 15cm pots? One plant will live in that size happily till they are ready to go into their final position (I use 10L flower buckets)
These are mine from last year, in may. I think at that point the roots were just coming out the bottom of those 8cm pots so they were potted up to pots your size.
I would re-pot the one in the middle though with like 10 plants. And as DD suggested bury the stem and pot right up to the first leaves.