[Gnits]

You need to take the car to a transmission specialist. They will tell you what you have. In the worst case your transmission might need a complete rebuild. In the best case it needs a new harness or topping up the fluid. But nobody will be able to tell you what it is without inspecting the car in person. And of course don't drive the car until the transmission is fixed, because you might permanently damage it even if it's possibly fully recoverable yet.

 

[Gnits]

Well, 3rd gear only means your transmission is in emergency mode, but not what the reason for it going into that was. And it can be anything from a dislodged connector to a catastrophic transmission failure, or anything in between.