Test: one free 40-min consultancy block, capped at three clients

I run a self-paced course and my leads come in waves, so I'm testing something cheap instead of building the big second offer yet. The idea is a free 40-minute consultancy block, but only for people who've already bought the course, and only three of them. All I'm really testing is whether the people already inside are my lead source or whether I'm kidding myself.