Dunes, weight, and HP are hell on belts. If you're not in the dunes, and leave your car stock, your belts will last a very long time (especially if trail riding). Even if you bump you HP's a bit you will still be fine. My belts generally last 1000 miles before I replace them (before they fail) and I'm 100% in the dunes running just under 200HP,,, but I'm pretty smooth on the throttle and run a VERY light car. In my experience the gents that run the sand in a 2200lb car, treating the skinny pedal like a switch on the dashboard have a harder time keeping belts on their cars than those that treat it like the analog device it is intended to be.
A few on the forum have just had bad luck with their clutches and had to have some work done on them to correct it. (see HP's post above) But those are not the norm in my experience.
The group I ride with most in the Dunes has several Sand Cars, several X3's, and several Po's. We are stopping 5 times to 1 for the Po's to replace belts to the X3's over the last few years. I've seen how they all drive,,, and I'd attribute this ratio to driving style and how they modify their cars over car manufacture. A couple of the X3's have never stopped for a belt issue (I'm one of those), one of the Po's has never stopped for a belt issue, yet we are all running the same pace. The lady in the Po that has never stopped is a VERY good driver, she is plenty fast, but very smooth.
One of the other guys and I are always comparing belt temps when we stop since we drive similarly, have similar 2 seat cars, and we are generally following one another in the group. (he is a bit harder on the throttle than I am, and I have a bit more HP than he does). My belt temps are always significantly lower than his, but his paddles are a bit more aggressive than mine, (his dig more, mine float more) and his car is quite a bit heavier than mine with mods/passengers. He's probably running about 400-500lbs more than me final weight. Even with these differences, we are generally the two that make it through a trip with no belt issues.
If you're running trails in your X3 and end up going through belts, you have an issue with the clutch specific to that machine,,, not the X3 in general. You made a good choice,,, I bet you'll be just fine.