Allow only specific apps to use cell data, only when on 3G

I use Tasker to sync every 30 mins when on 3G. I'm on a limited data plan, and the only apps I use constantly are browser, a Twitter app (Echofon) and Whatsapp. Is there a way to allow only these apps when on cell data, and allow access to all on wifi, either with Tasker or otherwise?

I've seen similar questions here, but they're mostly outdated (pre ICS) and/or involve restricting access using DroidWall, which does not differentiate wifi and 3G.