Hi Neik, I had a similar failure 2 years ago, a spring loaded arm with a bearing attached is used to help locate the gear selector drum to the correct position, (peg against a star shape) the bearing on mine fell apart allowing 2 gears to engage causing the 3 gear to fail.
if you remove the clutch cover and clutch it is easily visible, at least it's handy to check before having to delve deeper into the engine!

offending lever bearing


damage to gears