All three have solid black eyes and very small second dorsals. The thresher has an upper tail lobe almost as long as the body. Threshers and makos often free jump. So if you see a shark jump it is not always a mako. Put the polaroids on - get as high up on the boat as you safely can, and look constantly at and into the water around the boat.
Eternal vigilance is the price of spotting a mako, porbeagle or thresher.
They won't hang around like the blue sharks. You must pay attention. The tendency of most people when they see a shark is to look it over to get an idea of its size. Or try to make eye contact. Don't waste your time doing that.