I did mine on the GM 3800 Gen II V6 .. basically the same motor as the 3100 .. you do not need to loosen anything .. just put a wrench (long handle) on the tensioner and move it out of the way .. it only needs to move about 1/2 inch or so to put enough slack in the belt to remove the old one or install the new one. On my car a 1999 Grand Prix, the coolant overflow bottle gets in the way, but that is only 2 10mm nuts to remove and it makes seeing / threading the new belt much easier. At most, an honest mechanic should charge no more than 15 mins labor as that is all this job takes.
FYI: On that engine to relieve the belt tension, the tensioner must move Toward The Firewall (that is if the pulley side of the motor is on the passenger side, as it is on the Grand Prix)