The engine "works" less and less as you go up in gears so makes me think non "engine power" suggestions. 1st and 2nd would be the load gears and you have no problem? Also, if had never driven it before, maybe you are shifting into 3rd too soon. Bad clutch does not make sense, something slipping in trans? Good power in 1 and 2 seems to rule out engine. Not sure. What speed do you get up to in 2nd? What does engine sound like after shift to 3rd, revving higher than it's moving? shudder? bogging down? gradual speed build up?
