Yep, third babies are notorious for the prelabour thing. Mine did it to me. (Subsequent babies are likely to do the same.) It's tiring and frustrating, but it's normal, and your body will eventually get the "real thing" going and it will probably be fast. Hang in there! This baby will eventually come out!

I'm not quite as far along as you, but I've been having contractions off and on for a couple weeks as well. I know the feeling! JUST COME OUT ALREADY BABY!
All the best for a wonderful birth, hopefully soon!