Leaves turning dark. Help

Its my first grow ever. But I know what you mean, I'll probably make some LST , cause I have about ~60cm height only and about 1 meter in width . Anyway, I will try to veg them as much as possible. And I still hope that of those three plants 2 will be females, as only one had overgrown all others. If it will go as it was before, I hope to see their preflower in few days.

leave it to grow w/o cuts of the tops and a few days before flowering cut as much of the plant it takes , u may have to cut back the half plant so to suit in the box , 60cm's is very low so try to make a repot in a pot which is wide and not deep so to keep space for the green part of the [plant , roots will make theyre way towards the sides

Thanks for advice bro.
But I was thinking about LST after I repot. Or Maybe I will make a new growroom. I have another closet about 80cm height. I guess after LST it would be enough space.