missing 3rd...

Hello Everyone

I'm going thru some personal hard times lately with an ailing parent and on the wkends I try to forget my problems for a little and take the car out for a beating...
I wanted to if anyone knows why the car sometimes does not like to go into 3rd gear.
I've noticed it will do it at odd times no matter if I'm going slow (city driving) or at full throttle (In the city again) if I'm pushing it sometimes.
I just can't figure out why it does it sometimes 3 times in one day and then nothing for next 3 times I take it out.
Thanks to all who may have some input and excuse me if I don't reply asap.
Sincerely,
MAPS