Doesn't the batter (now runner)location depend on where he was when R1 missed 3B? If the batter had made contact with 2B BEFORE R1 missed 3B then it would be a double. If the batter did not make contact with 2nd base by the time R1 missed 3nd base then the runner would be sent back to 1st base and given credit for a single.
The trouble here is knowing where the batter was during the point R1 missed 3b. Most of the time both runners aren't seen so an assumption is made by the umpire that the secondary runer had successfully advanced before the missed base incident.
There was actually this situation in MLB last year but with runners on 1B (R2)and 2B (R1). R1 missed 3B and appeared to score, R2 came around and also appeared to score. The batter was now standing on 2B. The play was appealed saying R1 missed 3B. Appeal was granted. R1 was out, R2 was sent back to 2B because he of course never made contact with 3B before R1 missed it)and the batter was sent to first.
I'll never forget this because instead of having two runs, the batter having credit for a double and 2 rbi, everything basically went back to the way it was (runners on 1B and 2B) but now with one more out than before the play started.