|You could try VLOOKUP.|
My example uses the same sheet just so I don't have to deal with sheet names in the formula.
A B C D
1 Reg Name Program Contact
2 Bob P1 C1 See formula below
3 Mary P2 C2
4 Steve P3 C3
6 Att Name Program Contact
7 Bob P1 C1
8 Steve P3 C3
Put this formula in D2 and drag it down to the bottom of your Master List. You should see Attended for Bob and Steve.
=IF(NOT(ISNA(VLOOKUP(A2,$A$7:$A$8,1,0))), "Attended", "")
You can then do a Ctrl-c...Edit...PasteSpecial...Values on Column D to eliminate the formula and leave the text string Attended in the cells. Sort on that column and you'll have all of the attendees sorted together.
Since you need to do this for 3 sheets, you can either use three columns, one for each sheet or Nest 3 IF's together to check each sheet one by one.
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