I bought the starter pack in the game which came with a GTR3 class Mclaren (forgot which one). I primarily bought this pack just to play in GTR3 races but now when I go on online GTR3 races alot of times I don't see my one car that I bought in that class. It really is making me mad because that is the only thing I originally wanted and now I cannot even participate in GTR3 races. So can someone please explain what is happening?
It is up to the dedi hosts which cars they include and if they decide to leave one out of their selection (by mistake or on purpose) then you won't be able to join that server using that car. I think this is what's happening here. Try joining servers with the [RR] prefix, those are the official servers and should always include all cars of a class.
Ok so you want to drive the Mclaren, fair enough, but "cannot participate in GT3 races" is a little over the top as there are one or two other GT3 cars if you hadn't noticed.
Like @Christian Göpfert mentioned above, who ever setup the server can include and exclude any car they want, even if its advertised as a gt3 class server.