He hasn't ever gotten out of this one. I tried one of those comfort jacket/vest style harnesses and he could get out of that one. Then I had a figure 8 one that worked well, but I was worried about it snagging and the neck tightening up too much. This one is definitely my favorite.
I think it would probably be sufficient for a 3 lb rabbit. I never really realized how small his neck was until I looked at the harness post-adjustment.