As a suggestion, you could use:
Get(PersistentID) with v12
or Get(SystemNICAddress) with v11
If your iPad ID's are known, then compare the result with a value list of iPad 3 ID's. (You can obtain the ID or NIC when you register a new user.)
If the iPad ID's are unknown, you would need to add a "configuration" field (radio buttons) so that the users can set the type of iPad they have or alternatively base the type on the user account name. For the latter, you could use privilege sets to allow access to the higher res layouts. (These ways are not as secure.)
Thanks for the ideas Jade. I was able to figure out a webviewer method that seems to work too:
the devicePixelRatio is "2" for an ipad 3 vs. "1" for ipad 1/2 and passes that parameter to a script.