ok so iff i just leave it and dont top it will grow to high?
i fimmedd them when they were at 3 weeks
and theres lots of alternating nodes now reaching up to the light?
Your trying too hard, if you already FIMMed it leave it alone.
Topping creates more colas. In theory the higher up you top it the more colas you get. If you top it too high though you'll just get a bunch of little buds at the top of the plant as if you had FIMed it.
A plant actually has a maximum that it can produce whether it has one top or gets FIMMed and ends up with 10. You'll just get smaller buds.
BUT! It IS possible for a plant that is NOT topped to NEVER reach its full potential due to lack of area to grow.
My buddy would pinch off not cut just above the 3rd node with true leaves. That would almost always give him 4 tops with nice size buds and 4 stalks with shoots with mini buds. (1 1/2 - 3 oz per plant) depending on grow time.