If making the Repetition a numeric value is an option, then you could simply create the new column, and then go to Col Info, and under "Initialize Data" select "Sequence Data" and then specify a sequence of 1 to 3
Or you could create the formula
theList = {"a", "b", "c"};
If( Mod( Row(), 3 ) > 0,
theList[Mod( Row(), 3 )],
theList[3]
);
Make sure the column is defined as a Character column to use the above formula
Jim