When to transplant from RW to soil....

I have three ww seeds germinating in rockwool, I have just noticed the young'ins emerging from the surface. How long should I wait to transplant them into my pots, and should I introduce a 400w MH right at transplant?

You can bury the rock wool at any time; may as well do it now. Yes you can put them under the MH now, just make sure you have it far enough away so they don't fry. Providing you can keep the temperature under control, they should thrive under it. Happy growing!