When do you take a 3rd unit. I've hit a wall at 1000MMR and am trying to troubleshooting my issues and can't find an answer to this.

Lost 10 in a row tonight which was er... Well the first time I've wanted to throw my head into the wall over games since I was a kid.

I've realised I always buy a third unit every round if I can, paying 50 to do so. Some games I'll end up buying an extra unit every round.

Been watching Rooster a bit and I think he barely every grabs a third unit unless he needs it and haven't seen his opponents do so either. So clearly what I'm doing is far from the meta. I've never seen him really explain why.

Is there a hard and fast rule for when to buy a third. Is it just a case of never buy it unless x situation?