Can you service 3 PC's, 2 iPhones and 2 iPads on one service agreement?

Are you talking about consumer or business?  In either case the answer is yes, just different methods of making it happen.  The consumer side has multi-device options where you can buy a certain number of licenses to cover all your devices.  On the business side the licenses for different sorts of devices are separated out:
http://www.webroot.com/us/en/home/products/compare
and
http://www.webroot.com/us/en/business/products/smb

One family consumer.

In that case you'll probably want a 3-device license.  We don't actually have AV for the iOS devices, since Apple doesn't really let us that deep into the operating system.  We do have a secure browser for iOS though, for free.