If it came on suddenly, I stick with my contention that your linkage has loosened somewhere. What happens is your linkage get shorter because of the compression and it doesn’t go far enough for good engagement in third gear.
The fact that you’re not fully engaging the gear is why it’s making a grinding noise, And popping out of gear.
The turnbuckle adjustment is a huge pain in the a$$ if you’re doing it by yourself. I would make sure everything else is verified before you adhust the turnbuckle....
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