when?

I've got 3 plants 12 weeks old on the grow in my greenhouse in the garden. They are all 24" and doing well although 1 isnt as bushy as the others and is a paler colour. Do you think i should be nippin the tops around now? When should i start to force the flowering?.
Any advice?

 

I was thinking about making a sort of box around them using cardboard and then just putting a lid on the box every 12. The bottom would be open so the plants should be able to breathe ok. What do you think?