Bicycle Mechanics - Gear Shifting

I have a problem with shifting into 3rd gear with my Rhyno bike. I know its a cheapie, but I assume there is a way to fix it, as it worked perfectly when I first got it.

It also slips every once and a while. Any tips?

What type of transmission do you have -- derailleur (how many cogs) or epicyclic/internal?

Assuming you have a 3-speed hub gear, your return spring may have weakened, or your cable may be hanging up and not slackening sufficiently to allow the internal spring to pull the transmission into overdrive/3rd gear position. A little lube on the cable may help. Also, if you loosen the cable so that it hangs slack in 3rd gear, the hub should happily shift into 3rd gear and stay there until you retighten the cable. This is a quick test to determine whether the hub or the cable is causing your problem.

Thanks! I'm rather new at this, so I have not a clue what kind of transmition I have. I'll have to try what you suggested today.