I was browsing through the Accord forums when this topic caught my eye. While I'm not sure if the TL and Accord have the same manual transmission, you're experiencing the same symptoms as many Acura TL owners with the 6MT, including me. Acura dealerships have not been able to solve the notchy/sticky 3rd gear problem in the TLs either. However, many 6MT TL owners have switched their manual transmission fluid to GM Synchromesh Transmission Fluid - Friction Modified with great results. In fact, I just had mine changed yesterday.

When I first encountered the third gear problem (notchiness, intermittently hard to engage, even popped out of third gear a few times while driving), I took it in to Acura. The dealership encountered the problem once while test driving it but didn't want to have to break down the entire transmission since it only happened once in the three days they had the car. They told me to come back if it gets worse - not a happy solution in my eyes! I had them change the transmission fluid to their new improved MTF that was supposed to be better. The shifting was buttery smooth with no 3rd gear problems for about a month. The issues slowly but surely returned as time went on minus the popping out of gear problem (thank God!). After reading many, many stories of the same problem and the successes of others using the GM MTF, I decided to give it a try. $40 for three quarts and $20 for labor later, my shifting is awesome. I'm very hopeful this will be a more permanent solution but time will only tell.

Here's one of MANY threads that's been discussed by TL owners in recent years: http://tl.acurazine.com/forums/showthread.php?t=125763