The change interval shown in the computer is what the dealer would go buy if you want to do interim changes. In Canada, it shows every 2 years or whatever the mileage it calculates.
You live in Ohio, so I don't think you get totally insane temperature swings down there like we do up North here. My thought is that you are fine following BMW's guidelines.
Regarding doing it yourself, changing oil on the X3 is very easy if you've ever changed oil before.
One thing I'd note is that the front page of bimmerpost had a good post recently about a 2006 E90 325i driver putting over 200,000 miles on his car to date. The car still runs strong, required minor maintenance, and the poster said his average oil change interval was 17.5K miles. He lived in Maryland. Seeing stuff like that and some other cases on these boards leads me to believe that BMW probably knows what its talking about regarding its oil change intervals. Unless you see severe conditions (lots of short drives, extreme cold, extreme temperature swings, etc.) I wouldn't worry too much about cutting the intervals short. I will also say that I did an oil change on my X3 after 3,000 km to get rid of wear-in materials and after that I will do an oil change at about 1/2 the mileage the computer demands. If my mileage doesn't rack as fast as I expect, I will then do something time based (maybe annually following the winter). Again, in winter, we see severe operating conditions here in Calgary.