is my plant to small for 3 weeks ?

I am growing her in rockwool with 400w mh has a few sets of leaves on of the leaf sets have five points but is about 2 in tall

It is a little small. And it looks hungry. Are you feeding it?

Very small for 3 weeks

Looks like she needs nitrogen. rockwool looks soaked so roots not getting enough oxygen to grow her I would just transplant into soil with a really good amount of perlite in bigger pot and lightly water round the stem give it time to let's roots set in and grow out the rockwool. Smaller plants like small amount of water but more often and bigger plants like more water less often basically water till small amount of run off when soil about inch dry on top water again

I grow 4' plants with 400W. You're growing seedlings. I would seriously consider putting them under a couple CFLs for a week and let the roots catch up to the foliage. You have way too much light for those little gals. @youngg is on to something as well, I hate rockwool and think that shit should be straight banned from weed growing. Its for insulating houses and soundproofing recording studios.