I don't think 3 hours is enough for that. Before styling the hair, the stylist usually flat- or curl-irons or blowdries it in some way first and doing that already takes a little while, especially if you've got someone with wavier or curly hair. Then there's the actual styling that'll take a while if you're doing an updo, especially if the person's hair is longer... I would allow about 4 hours for everything if you have a few people working at the same time.
I had 2 hair people and 1 makeup person. We did 12 hair and 10 makeup and it took us a little over 5 hours. In my case, I had a couple of with short hair. Some of my girls did updos, and others wore their hair down.
Hope that helps!