I don't think the Netflix app supports AirPlay. However, it does support wired video out so you can use Apple's Component AV cable to connect your iPhone to any TV that accepts 480p component inputs. I've used my iPad 3G in just such a manner (both over WiFi and 3G).
In any case, you can't use 3G with AirPlay because you need to have WiFi enabled for AirPlay to work and you can't have both 3G and WiFi enabled at the same time.(*)
Here is a list of AirPlay enabled apps:
http://theapple.tv/apps/list-of-airplay-enabled-apps/
Note, however, that in my hands several of these apps do not work reliably over AirPlay.
I have had success with the following AirPlay-enabled apps from Apple (once again, only for WiFi):
iPod (media player for iPhone/iPod touch)
Video (video player for iPad)
YouTube
Photos
Safari
As for Safari, most web sites do not work with AirPlay and many that are reported to work only work intermittently (or unpredictably).
I also have had success with AirPlay on the following third-party app but I'm not certain whether any of these support video out with Apple's component AV cable:
Newsy (not 100% reliable over AirPlay)
Pogoplug (only works when paired with a Pogoplug device -- which is a type of file server/NAS)
StreamToMe
PBS (for iPhone/iPod touch, NOT the "PBS for iPad" app)
(*)Perhaps if you had a tethering plan with your iPhone you could use the phone's 3G network to supply a WiFi connection to the Apple TV and that
might work with Netflix. But, I'm fairly certain that AT&T's tethering plans have fairly low data caps and you'd have to pay extra for tethering anyway. In any case, it would be cheaper to go with Apple's Component video cable and I know for a fact that the AV cable will work with Netflix over 3G.