I tried the Incipio Dual Pro since I had that for my G2. It really added too much in size to the phone and harder to hold comfortably.
I like belt clips so was going to order the Seidio, but remembered Best Buy has their Platinum series cases which are exactly the same,
but for less money. So I bought the Platinum Series Black case for the G3 w/belt clip for $39.99 ($54.99 from Seidio).
The case is much slimmer and holds very securely in the belt clip. I wish there was no kickstand, but its not really in the way.