Hi PeterFri,
Try this
1.Select cell range on sheet1 and go to Home > Conditional formatting > Use a formula to determine which cells to format
2.Enter this formula there:
=COUNTIF(Sheet2!A$1:A$5,A1)>0 and for sheet3 =COUNTIF(Sheet3!A$1:A$5,A1)>0
3.Click on Format > Fill > Red
4.Click on OK > Apply
Let us know if you need further assistance.
Best Regards,
Waqas Muhammad