My car, oddly enough, had no slippage whatsoever before it stopped shifting into 3rd. That's why I'm not so sure that it's a purely mechanical problem...
The car itself has 154k miles on it (247k km). For the record, Dad and I have replaced transmissions before---we replaced on in a 1996 Ford Aspire and one in a 1988ish Chevy Blazer. We know what we're doin'

. lol
I will not get assistance in purchasing another vehicle. I know this, but I'd rather not ellaborate on it...Let's just say that it's not quite a viable option to take the thing to the scrap heap. After all, it never slipped beforehand.
I would sooner replace the entire main drivetrain (engine AND transmission) than junk this vehicle. I AM qualified to do that.