How early can I fim/top my plants?

I have 4 plants growing right now. 3 of them have just started sprouting their 3rd set of leaves and the 4th already has. I was just wondering how much longer i should wait before i try topping or fimming them. I only plan on doing this to 2 of my four plants, so i can see the difference for myself.

If they look ready to you then do it. I waited 3-4 weeks before doing mine, they have about 7-8 nodes. I could of done mine sooner tbh