Google Sheet - Detect duplicates across two sheets

[parvares]

Hello, I have 2 Google sheets (Sheet1 and Sheet2), link here. They both have numbers in column A, starting from cell A3. I would like a formula that finds duplicate values in entire columns of both sheets. I found this formula googling, but doesn't seem working:

=(COUNTIF(A:A;A:A)>1)+(REGEXMATCH(A3;TEXTJOIN("|";1;UNIQUE({INDIRECT("Sheet2!A:A"))})))

Thanks for helping me!

 

[parvares]

Update. I found this formula applied to two sheets:

=ArrayFormula(UNIQUE(TRIM(FILTER({A3:A;Sheet2!A3:A};COUNTIF({A3:A;Sheet2!A3:A};{A3:A;Sheet2!A3:A})>1))))

Could you tell me how would it be the same formula applied to three sheets?
Thanks for helping me!