Actualy, I just brough the car back from the cotage tonight - so I just did 40 miles in 2nd gear!
Man I really tought I had found my problem tonight when I took off the splash guard behind the left front wheel, to get access to the revolution sensor on the tanny... There was a ground wire with a plug at the end hanging loose just beside the suspension. This wire was connected to a terminal under a bolt, on the side of the transmission housing. I put a new terminal on and re-installed it... Erased the code via OBDII, disconnected the battery for 30 minutes... then went for a test drive... same thing again... no 3rd gear...
Couple things I noted on the way back... Trans just don't want to get into 3rd. (and 4th off course...). When I'm in 2nd gear, if I take the O/D off, the transmission disengage and the engine revs... Putting it back ON locks 2nd gear properly again.
When going down hill, if I let the trans in D4, the rpm will just fall back to +/-1000RPM. If I take off the O/D then, there's a little bit of compression/decelaration from the engine which rev up a little...
One other thing, I checked the revolution sensor before bringing the car back tonight - there was some sort of oil on it and very small metal residue (like on the transmission pan magnets...). I cleaned it and reinstaled.