It happens quite often on 6M Hondas, like RSX, Si, TSX. Each gear has a part called 'synchro', that locks into the shift into each gear.
If the synchro on 3rd gear worn out, it won't lock tight the shift and kick back the shift action.
Come over early tomorrow (Saturday), like between 9:30-11:00 and we will help you out as much as we can.