3rd gear crunch on transit 280 fwd?

hello i have a 2l 280 transit fwd i have just bought the guy i bought it of would only let me test drive it in his yard so only tested 1st 2nd and reverse gear van looked and drove mint (or so i thought) so i bought the van drove off as soon as i tryed to get 3rd it crunched (only minor thow) went to 4th no probs then 5th no probs but when i then came down the box 5th to 4th was no probs but from 4th to 3rd is near impossible to select its that bad i have to miss 3rd gear coming down the box 2nd to 1st no probs either it just seams to be on 3rd gear (coming down the worst) ive been reading all day and carnt make up my mind on what to do as i dont think its the cables as its only 3rd gear thats the problem so it must either be the selector or bad news for me the box its self ? please any info would be greatly recieved as i dont fancy being bent over