LOOKUP function for multiple criteria

I have tried many different formulas to make this work, and this is the closest one I have come across, but I am still having issues

=LOOKUP(2,1/('Master PO Schedule'!$C$26:E$67=A4),'Master PO Schedule'!$A$26:$A$67)

What I am trying to do is find cell A4 value on the Master PO Schedule (they are in random order, all throughout the range specified), and then return the row header from the Master PO Schedule (Column A - date). This works for items that are populated in Column C, anything that has been populated in Column D or E gives me back #N/A. If I change the formula when looking for the value of A5, which has been populated in Column D, to =LOOKUP(2,1/('Master PO Schedule'!$D$26:E$67=A5),'Master PO Schedule'!$A$26:$A$67) I get the value I require.

I have several hundred values that need to be looked up in this manner, and I need to know the date that they have been scheduled without changing the formula on every line depending on whatever row that they appear. Is there a way to add an IF, AND, OR function, to look up all three rows at the same time? And return the result (date) under which it is found?

I have tried INDEX & MATCH, a VLOOKUP, I have the data horizontal, and well as vertical, and I always get stuck at this point.

Array-enter (enter using Ctrl-Shift-Enter)

=IFERROR(INDEX('Master PO Schedule'!$A:$A,SMALL(IF('Master PO Schedule'!$C$26:$E$67=A4,ROW('Master PO Schedule'!$C$26:$E$67)),1)),"")