I also have two mares and a gelding. The gelding has a "dominant" personality. The one good thing about my situation... my thoroughbred is"alpha" mare in the pasture, and because she is 17 hands, the gelding (who is 13.2 hands) never really stood a chance of beating her up.
I started the gelding in a pasture next to the pasture with the mares, so that they could smell each other, and just get used to having each other around. After about a week I did let the gelding out. My thoroughbred snaked her head at him one time, and he didn't even think about questioning her. My quarter horse though... went straight to the bottom of the herd. And that is where she has stayed. At first our gelding did assert his dominance over the quarter horse often, but never enough that I worried he would hurt her.
In the past I did own a mustang gelding who was terrible with mares. It was at least a month before he could be turned out with the arabian mare I had at the time. And even then he ran her so hard that I seperated the two permanently.