I recently saw a 50' Scarab with triple Yanmar's into TRS drives. The owner claims the boat will not exceed 30 mph. I was thinking "what if" the wrong rotation would be the culprit ?

Not enough info.

More than likely, totally wrong props. Rotation alone as the problem is not likely.

Which way are the props supposed to spin on port, starboard and center? In other words lets say the outer drives spin in...which way should the center drive spin?

When I had my triple, I had two props spinning right, and one spinning left.
I would think the props are wrong on that scarab.

Which two props were spinning right?

On mine the center and the starboard were spinning right and the port was spinning left.

I rember seeing an article on a big diesel boat years ago. They couldn't get it to perform very well. They started to expirement with props, and it really woke it up.
I think props probably is the problem with that boat, but I think you would need to talk with a prop expert to know for sure.
With a little more information you could use a prop slip formula to see how the current props are performing. That would help get you started in the right direction.

Thanks GS