if mixer 3 isn't latching/stopping, you may have trouble getting score advance beyond the second level and extra balls.

it is possible to remove the mixer latch arms without moving/removing the mixer shaft or mixers.
- Unhook the four springs that pull the arms up
- push the top of the arms towards the back door out of the slots in the top plate
- remove the screws on each end of the shaft the bottom of the arms pivot on. You may need to unscrew the coil and remove the plunger to access a screw
- you can usually drop the arm assembly out the bottom

see these for an idea of what's in there:
https://bingo.cdyn.com/techno/commonproblems/wear/mixers.html
https://bingo.cdyn.com/techno/overhaul/magic_ring/mixerlatch.html

note those pages talk about a game with 5 mixers instead of 4 and there's a mixer latch coil on each end of the shaft, but the assembly is basically the same.