When you say it revs high... is it just staying in 3rd too long, or is the engine revving away like the transmission is in neutral?
If it is just holding 3rd gear too long I would start by checking the linkage. Actually I'd do that anyway because it's so easy. See this video:
http://www.youtube.com/watch?v=z2tLAwJbpYg
You just adjust the cable length so that it is right at the "no tension" point with no throttle application.
-J