Runner Abandons Base on Ground Ball

 
With the bases loaded and one out, the batter hits a fair ground ball to the third baseman, fielded about 1-2 feet from third base. After the third baseman fields the grounder, he throws to first base to easily retire the batter-runner (B1) -- but surprisingly WITHOUT first touching third base -- where he could have easily forced out the runner coming from 2nd base (R2)
 
The runner that started at third base (R3) mistakenly believes that his teammate behind him (R2) was forced out at third base (for the 2nd out of the inning) and R3  mistakenly thinks the easy force out of B1 at 1st base completes the inning-ending double-play... so, R3 runs only halfway to home and then runs into his third base dugout!
 
Meanwhile (and you cannot make this stuff up!), after B1 is easily retired at first base (for the second out of the inning), the defense throws wildly attempting to retire R1 near second base. In the end, both R1 and R2 score.
 
How do the umpires sort this out?
 
Call
When a runner obviously abandons his effort to touch the next base, he is out (NF 8-4-2 and Casebook 8.4.2C). While this is an unusual play, since one of our umpires reported it, it is worth taking a look.
 
R3 is out when (in the judgment of the umpire) he abandons his effort to advance to home. When he entered the dugout, he should be declared out. Since this is the third out, the inning is over. No runs score.