Mycoplasma genitalium is cured 71% with 21 days minocycline. 100mg x2 daily. wouldn't 100mg 3x daily increase cure rate and also a 30 day duration?

That's not how it works. The tetracyclines (doxycycline, minocycline, and a couple more) never are reliable by themselves against M. genitalium. Sometimes they work, but 7 days is enough; effectiveness does not improve with longer treatments. However, these drugs can be used for a week BEFORE then taking a week of moxifloxacin. It's a confusing issue. Best to see an expert, like an NHS GUM clinic.

Too much of a good thing is not necessarily a good thing. what you suggest is 'overkill' and can be harmful in toxicity and other side effects.