Yes exactly. This is how I understand it at least
So lets continue your scenario: We multiply it with the GFR so: Freely filt. subst= 3mg/ml GFR=120 ml/min.
Fil.load= 360 mg/min. So concentraion in both compartments stays the same, but now u know per minute there is a total "transfer" of 360 mg. I hope it is clear to you know. Good luck with the studying.
by the way: I think this is an excellent question