The Hummer H3 is a fun truck to own and drive but like all other vehicles, does require routine maintenance. And don't expect to see good gas mileage. H3's typically get 15-17mpg. Depending on what the previous owners have done or not done to this particular vehicle could be a big part of what needs to be done going forward. I would have a reputable mechanic look it over real well so you know what you are getting into.
The Hummer H3 was fine for me because I tend to do a lot of my own maintenance and except for major things, my own wrenching. I put both of my kids in Honda Elements that I previously owned when they were seniors in high school and 6 years later they are both running strong with well over 100,000 miles on each and no major money spent. Those cars were roomy, versatile, had all wheel drive, and required minimal maintenance. They also averaged 22-25mpg.

 

I wouldn't buy an H3 and expect it to be low maintenance like a Honda or Toyota, especially if you don't plan on doing any repairs yourself.