Tag up at third

[mdntranger]

I'm going to assume that the OP means that R1 was sliding back into 3rd base after the catch was made (i.e. wouldn't have made it home regardless).

Initially, you'd have to ascertain whether the players were making an appeal on leaving early or a play on a runner off base (the no accidental appeal discussion). If the former, you weren't able to see whether they left early and therefore the initial call would be 'safe' pending any further discussions/information. If the latter, unless a tag was applied before the runner slide into 3rd, also 'safe'.