Sounds like gearbox wear to me I'm afraid, probably on the 2nd gear synchro hub, these tend to have a hard life if you change gear to quickly. Its probably worthwhile checking the linkage but if it feels as though its going right in, it will be the box.

Worn synchros tend to make engaging a gear (esp downshifting) difficult. Agree that 2nd synchro leads a hard life, but third as well?

It could be that the selector forks are bent and not allowing the baulk rings to fully engage. If you're very lucky, a maladjusted linkage would have the same effect!

The gearbox is controlled by cables which are mounted below the gear lever. It's possible to adjust these but it's definitely a garage job because you have to get right underneath the car to be able to do it.

I rather think this might be your problem. There's nothing fundamentally wrong with the design of that gearbox; mine's done 190K and it's still going strong!