Michael Grahek identified a NEW entry into the record for 3
baserunners caught stealing by a catcher (3 CS Assists). I had
already identified this being accomplished by two other backstops:

Jocko Milligan PHA (AA) 7/26/1887
Les Nunamaker NY (AL) 8/3/1914

Three Assists in an inning by a catcher (not all CS types) have been
accomplished five times by:

Ray Schalk CHA (AL) 9/30/1921
Bill Dickey NY (AL) 5/13/1929
Bruce Edwards BRO (NL) 8/15/1946
Jim Campbell HOU (NL) 6/16/1963
Jim Sindberg TEX (AL) 9/3/1976

The above seven "three assists in an inning" have been verified via
ProQuest searches. A 1980's Sporting News indicated that there had
been 10 such events. Now that Michael found the 8th (Johnny Kling
NYG v. BSN 6/28/1912) that leaves two more to be located.
Unfortunately the TSN didn't list all of the catchers and/or dates.