I have a 98 GTP identical to yours. I accidently drove about 18 miles in 3rd gear.. (not paying attention) luckily i caught it in-time before serious damage
Just because you drove 18 miles in 3rd gear doesn't mean it's going to do damage, on the highway, yes it could, but if your doing city driving and going under 50 mph it wouldn't hurt anything
97 GTP - POS parked in backyard
07 Saab 9-3 2.0 turbo 6-speed - Awesome