Try

C3=IFERROR(INDEX(INPUTS!$E$2:$E$18,IFERROR(MATCH(OUTPUT!C$2&OUTPUT!$B3,INDEX(INPUTS!$B$2:$B$18&INPUTS!$C$2:$C$18,),0),MATCH(OUTPUT!C$2&OUTPUT!$B3,INDEX(INPUTS!$B$2:$B$18&INPUTS!$D$2:$D$18,),0))),"")