The two grip fence stretcher is all you need if you are just doing a few wires now and then.
The three grip will help grip the wire after you have jacked the stretcher all the way in...therefore giving you continuous stretching without re-setting everything. This three grip stretcher is un-necessary until you are doing a bunch and need to save a few minutes here and there.
If you already have a come-a-long, you can also stretch wire with a Speeco Wire Gripper from Tractor Supply. You do need an anchor to stretch from though, like an end-post or corner-post. You can also use two Speeco Wire Grippers...one at each end of your come-a-long.
With all of that, I use a plain two-grip wire stretcher for 90% of my fence repair, but I keep several other tools like the Speeco handy.
I also have a Maasdam Wire Grip which I hate because it never seems to fit into the tight space you need to stretch and wrap a wire.
http://www.bing.com/images/search?q=speeco+wire+grip&id=D5DF8A6BAB3B0BEAD35B0C2B7AF28A0B906FE93C&FORM=IQFRBA