Hello and welcome to JustAnswer!
What happens when it should shift into third? Does it rev up like neutral and then go back to second?Did the problem start suddenly, or had it been slipping into third for a while?
If it happened suddenly then you have a broken overdrive clutch hub or planetary geartrain problem. It's very likely to be a broken hub. The overdrive clutch hub is splined to the overdrive clutch discs and has a shaft the transmits power to the geartrain in third and fourth gear.
If the problem started with it slipping into third gear then it's going to be a failed overdrive clutch. Either way the trans will need to be disassembled for repair.