Firstly, there is absolutely NO C O N F I R M E D EVIDENCE that GW3 exists, yeah there was the chinese article "leak" and other things, but GW3 has not been confirmed, heck, EA might just call it quits with PvZ and not do anymore spinoff titles and focus on the main series, nobody knows.
Secondly, there could be GW2>GW3 data transferring, but seeing as EA is in control of Popcap they could easily just say nope! and not do the data transfer stuff this time around, but that's assuming GW3 even exists.