2000 Chevy S-10 transmission stuck on third gear

There could be many reasons for this, so you obviously want to have it looked at by a trans shop (most dealers aren't even that good at diagnosing trans problems, since the automatic transmission is extremely complicated). A simple check may be to inspect the linkage on the trans to see if there is a jam. Does the car work in reverse? Is the engine light on or flashing? Some cars are equipped with a "fail safe" mode, in which the car will only work in one gear when the car's internal computer (ECU) detects something is wrong - sometimes it's called "limp-home" mode. If there is a fault stored in the ECU, you may not be able to get out of third gear until the fault is cleared (you need to have a diagnostic code reader to have it cleared). If there is a code, the diagnostic reader will be able to clear it, BUT you must repair whatever the computer says is wrong so that it does not happen again. If you don't have a diagnostic reader, you don't have to buy one, some places will offer free checks (I think AutoZone does).

If it's not a fault code. Well, lets just hope it is. Check the fluid level as well, and note the color and smell of the fluid. If it smells burnt, and is dark brown or black - there may be worse problems. Like I said, most professionals have a hard time with automatic transmissions, let alone home mechanics. Take it in, but get a couple of opinions before you throw money away. Good luck, and let me know what you find!