>What is the difference between a PW FlexTT5 and PW Plus III?
The PW Plus III transceivers are Manual triggers and only support Manual flash control.
The PW FlexTT5 transceiver has Control TL technology to provide i-TTL RF wireless flash control when used with another FlexTT5 or a MiniTT1 on camera. In addition to i-TTL flash control, the MIniTT1 and FlexTT5 maintain Manual flash triggering capability when combined with another FlexTT5, or any Plus II, Plus III, or Multi-Max transceiver.
>Also how does the AC3 fit in with all this?
The AC3 is mounted in the on camera MiniTT1 or FlexTT5 can adjust the flash exposure compensation for up to 3 remote groups when using RF wireless i-TTL flash control with Speedlights from the camera position (just like the Nikon CLS wireless flash control using commander mode). It can also be used to adjust the power level of strobes (Alien Bee, Einstein, White Lightning, and Elinchrom) from the camera position when used with the appropriate adaptor. It works very well in bright sunlight and other conditions where Infrared wireless flash control won't work.
To take advantage of i-TTL flash control you will still need: SB-600, SB-700, SB-800, SB-900, and or SB-910 Speedlights and a MiniTT1 transmitter or FlexTT5 transceiver on camera and a FlexTT5 for each remote i-TTL Speedlight. Good Luck and Enjoy your Nikons.