I would strongly discourage using a bike carrier attached to the trunk, if that's what you meant. Most especially to fit three bikes. You will most likely end up damaging your car's paint.
I had a trailer hitch installed on my '00 9-5 Sedan and bought one of the cheapest bike racks around (good quality for around 120$ canadian), although it only fits two bikes. The hitch is a bit expensive at first (around 230$ canadian if I remember correctly), but a hitch rack is much more convenient than any trunk attached rack, trust me on that one.
If you go with the hitch solution, make sure to purchase a strong rack for holding your three bikes (i'm assuming it's three adult bikes), cause I heard that fitting more than 2-3 bikes on the rear of a 9-5 is pretty much the maximum load you can go without risking damaging your bikes on the ground, because of lever effect.