3 plants in that area is pushing it, you will need to flower them while still quite small, probably 4 weeks veg max, maybe 5 at the very most..
3 x 20cm round pots is probably going to be all that you can fit, make sure they are at least 25-10cm deep.
3 x 25cm square pots would be the go if you can find them, again at least 25-30cm deep.
18/6 is the best lighting schedule, they won't grow any much more at 19/5, - save your electricity bill, - plus they need adequate sleep!
Hope this helps, Organoman.