Mine sometimes has a problem coming out of 3rd. I have to stop trying to change for a second then try again and it's fine. It's ok when my gearbox has enough oil in it, but it's leaking .
Have you topped up the oil in the box yet ???
Also I was told it could be sticking part of the linkage, can't remember waht part it was though
Have you tried synchronising gear speed to engine speed as you come down the gears? If this seems to work, your third gear synchromesh is probably buggered. If not, the gremlins have been under your car in the middle of the night. Buy a shotgun...