From what I can tell from the specs, the 3955 doesn't support wifi out of the box. However, it does have an SD slot, which means you can get an SD wifi card. Before you do that, however, you need to make sure the operating system is 2003 (instead of 2002), which means you might have to update the ROM first. See these threads for details:
Help ROM UPDATE IPAQ 3955
Wireless card for iPaq 3955? - Mobility Today
Also see Ike's very useful tutorial on wifi:
http://wifi.aximsite.com
It was written for Axims, but still contains a lot of useful general info.