1st grow: question on transplanting

My first grow - at 3 weeks, pic below.
Seems to be going well so far. got a heating pad under the plants to keep em warm. humidifier underneath that, using CFL lights.

my question, that I had trouble finding the answer to, when is good time to transplant to a bigger pot. as you can see I have them in 5 inch party cups.
- is it too soon to transplant?
- when I transplant normal plants in my garden, I use a root stimulating additive, is that OK
for these plants?

any help, greatly appreciated

 

Thanks