Mugen's opened up the 1400mAH battery for the Pre3 for pre-order.
You can find it for $44.95 at:
[HLI-PRE3SL] Buy Mugen Power 1400mAh Extended Battery for HP Pre 3
And if you use the coupon code PRE3, you can get 10% off.
The regular Pre3 battery is 1230mAH, so you're (theoretically) getting 13% more battery life with this Mugen.