with the monkey speed mod, I get readings fluctuating between 174% and 175% with all three on.. they do stack, and I can easily catch alliance on their run to ratchet after getting beat down raiding XR
I 2 box, and have tested them on different characters on both of my accounts, with none, with one piece, 2 pieces, and all 3 pieces.
I recommend that anyone who is serious about using these pieces search curse-gaming for the mountequip mod, which swaps up to 5 pieces of gear automatically when you mount or dismount.