How to combine 3 sheets into one using QUERY(IMPORTRANGE)?

As mentioned here and the comments alike, you should:
First authorize the function IMPORTRANGE and then use it within the QUERY

Also, since you stack open end columns (A2:O) instead of closed end (like A2:O33) it is a good idea to constrain the results by using something as simple as WHERE Col1<>'' in your query.

Summarizing, your query would be:

=QUERY( 
{ 
IMPORTRANGE("11Vm7p_0Tx_DI649kdrHuxMaK5wauuI281175hfwBqRo", "'Termination Review'!A2:O"); 
IMPORTRANGE("11Vm7p_0Tx_DI649kdrHuxMaK5wauuI281175hfwBqRo", "'2nd Week Warning'!A2:O"); 
IMPORTRANGE("11Vm7p_0Tx_DI649kdrHuxMaK5wauuI281175hfwBqRo", "'In Review'!A2:O") }, 
"Select Col1, Col2, Col5, Col6, Col8, Col11, Col12, Col13, Col14, Col15 WHERE Col1 <>'' ORDER BY Col1 ASC",1 )