R3-A2, according to canon, was simply present at Echo Base during the Imperial Attack.  This biography is barely edged out in detail by the Legends Wookieepedia entry, which adds that R3 was able to calculate military attack approaches and store 5 hyperspace jumps.  Oh, and that he had a round eye, which just sounds racist.  Other than that, he’s just there in the background of a scene or two, which makes it a great candidate for an action figure.  He can be spotted standing in the background behind the fuselage of an X-Wing in the hangar, as Major Bren Derlin and Lieutenant Cal Alder discuss how they are going to finally murder Luke Skywalker, once and for all.  Maybe.  Because if you read through the details on Angelfire, one of the best resources for identifying astromechs, you might walk away thinking this figure isn’t actually properly painted as R2-A3. 

The clear dome is, in fact, clear, unlike the cloudy version that was used in creating the R3-M3 build-a-droid.  A much crappier version of R3-A2 was included as part of the Battle of Hoth 4-pack from the Saga line in 2003.  The release of this build-a-droid version contributes to making that set utterly obsolete, as every component has since been updated and improved.  Well, except for maybe the tauntaun gutsac.  That’s still cool.

What you need to build R3-A2:

BD16 Luke Skywalker

BD29 R2-D2

BD30 Han Solo

BD36 Boba Fett

BD31 Chewbacca

BD37 Clone Commander Deviss

BD32 Imperial Scanning Crew

BD35 Padmé Amidala