I'm no "towing expert" but if you feel the V70XC couldn't handle it, the XC90 might not either as it's almost the same drivetrain. The XC90 uses a S80 platform, S80 engine and trans except that the V70 Haldex AWD system is then added on, like the XC70.
The vehicle wasn't made for heavy duty towing like a big domestic truck and I've never heard of Volvo ever offering a "towing package" with added engine, trans coolers. The trans has no abilities to manually lock out OD either.

A good new car salesperson with alot of product knowledge should be able to give you more specifics but I doubt the XC90s been tested for long term towing with a 3000 lb load to date.