Generic specs on a BMW X3 say maximum towing weight is 4,400 lbs. But, it's best to not get close to that much weight in an a travel trailer. You've got to deal with things like the cargo capacity of your SUV, tongue weight, and cargo capacity of a TT which limit how heavy of a trailer you can tow. The more weight you carry in your SUV and/or in your TT the lower the weight of a trailer you can safely tow.
The X3 comes with a 4 or 6 cylinder engine. I'd want the 6 for any towing because it has more hp and torque. You may be limited to a light popup (camper trailer) or a very small ultralight TT.
Here's some links that may help you calculate how heavy a trailer you can safely tow:
Here's where you can download a weight calculator: