I'm making a little robot from some cheap omniwheels with a friend using S04NF continuous rotation servos and 3d printed frame. The frame is a regular hexagon with attachment points for the servos in a triangle configuration. Also included are press fit mounts to connect the omniwheels to the servos. Our omniwheels are very similar to these hobbyking ones but have an inner diameter of 8mm rather than 10mm. It's easy to adjust the oozby file to whatever diameter you need.
The M5 bolts in the picture aren't strictly necessary. It press fit's quite firmly on to the servo mounting tabs, but for extra security you can loop a zip tie through each of the bolt blocks behind each servo tab and tighten it down with some pliers instead of bolts. You can also change the diameter of the servo tab holes to be smaller and screw the servos on to it.
print two bot-plates and three servo mounts. I used PLA - ABS isn't as stiff so it may not press fit as firmly, but a few zip ties or bolts will do the trick.