Honda Accord V6 6MT, doesn't always go into 3rd gear?

Hello, This indicates one of two things: a worn bearing or a bad synchronizing ring. Both of these require the transmission to be removed and rebuilt to some extent. Alternatively, you may have worn bushings on the shifter or where the shift cables connect to the transmission. Those can be identified by shaking them back and forth while watching for play. Hopefully the bushings on the shifter are the cause, as they often are.