I have these values in sheets.

Sheet2!A1 = 5
Sheet3!A1 = 7

Now, I put Sheet1!A1 = "Sheet2" which can be varied to "Sheet3" or "Sheet4"
etc.
User has to provide the name of the sheet here in Sheet1!A1.

The problem is what formula to put in Sheet1!A3 so that it checks the value
in Sheet1!A1 and depending on sheet name, takes the
value of A1 of that sheet.
Ofcourse it is wrong, but, something like this [Sheet1!A1]![A1]
i.e. if I write Sheet1!A1 = "Sheet3", the value in Sheet1!A3 becomes =7.

Any help is appreciated.