Okay so my boyfirends Citreon AX third gear keeps slipping out when he puts it into 3rd, l am guessing it is probably the Synchronizer, can you just buy like one or do you have to buy a set or change the whole gear box? and how much do these cost?

It is better to go throught the entire gear box. There could be more worn or damaged parts. Besides either way the gearbox must be removed and disassembled. It is normally cheaper to rebuild vs buying a new one. That is unless you get a used part.