Anywhere in those three counties you are going to have a really hard time finding a job. Even minimum wage jobs can be pretty scarce. Even with a college degree it took my wife a year and a half to find a job that paid above minimum wage in Mendo, and it was in an entirely unrelated field.

Mendo is going to be the most expensive place to rent/buy a place, and has the most stringent and restrictive medical regulations in the entire state. If you are really just doing a personal grow, you probably do not have to worry about it. Mendo is also closer to bigger cities.

Humboldt is cheaper, but it is also very isolated. You have to drive at least 3-4 hours in any direction to get to a real city. The weather is also really horrible (rains all the time, very little sun) unless you are in Eastern or Southern Humboldt. Medical restrictions are also more liberal, unless you are in Arcata or Eureka. Humboldt has the biggest social scene, especially if you are near the University.

Trinity is the most undeveloped, but it is closer to Redding so it is not quite as isolated as Humboldt. The biggest town is only about 3,000 people, and it is the most rural of the three counties. Finding a job will probably be the most difficult here, since there is no real industry to speak of. I'm not sure how the medical regulations are there. Rent and land is going to be the cheapest here though.

You might also want to look into some of the areas in the Sierra foot hills. El Dorado county in particular has been attracting a lot of 420 friendly folks lately it seems like. Land and rent are much cheaper there as well, and you may have an easier time finding a job since you will be close to Sacramento.