NS-3 provides models for 802.11a, 802.11b, 802.11g and 802.11n physical layers.

see: http://www.nsnam.org/docs/models/html/wifi.html

Will NS 3 supporting WSN too?

You can use the NS-3 to set up basic wireless environment, and then by adjusting the parameters to make it as close to the WSN. In fact, TOSSIM is more professional WSN simulation tools.

I am not familiar with NS3 ,Does it similar to 'NS2 '?

there is a big difference between ns2 and ns3. If you have a working knowledge of ns2 I would suggest you continue with that but if you are going to start learning ns2 from scratch then it is better to learn ns3 instead of ns2.

Regarding 802.11a/g implementation in ns2, yes implementation is available in ns2.35. Implemented files are mac802.11EXT.cc and Phy802.11Ext.cc etc.

Here is also one paper about that you can refer to: Overhaul of ieee 802.11 modeling and simulation in ns-2. http://dl.acm.org/citation.cfm?id=1298155

Good Luck