Hi guys, my son's mk2 05 punto 1.2 won't go into 3rd gear! When the engines off we can get 3rd no problem but when driving we can't get 3rd changing up or down! All we get is a crunching noise! Any ideas what could be wrong please

3rd gear synchro gone and now the teeth have rounded off and won’t go into gear. Needs strip and repair. Cheaper option is get a gear box from a running car/mot failure and swap. Mk2/2b will fit. Grande Punto has different ratios .