Should I top my Northern Lights?

my tip first train your plant safe and use LST toping = +time

Yep! top them, followed by LST. Once she starts getting really bushy any leaves on top of each other remove gradually.

I mean first grow i would just let her do its thing look how it grows and so on before going for training or "advanced" growing methods. Since you already asking about that i guess you did your homework altleast a bit wich is good. Thing with topping is that you often need to do some Defoliation and/or LST afterwards wich can get confusing or even stressful the first time you do it. Since you got 3 you could even top one, LST the second and let the third do its thing. In that way you get the most knowledge and training from your first grow for the future.

I would. Even if a plant naturally grows short and bushy it can still benefit from breaking it's apex dominance. Two tops are better than one. Some strains perform better topped than others though. Never know until ya try.