3 variables to fill up one destination

Dear Excel gurus,

I need help to figure out on how to solve this problem.

Here is the scenario:
Sheet 1, Cell A7 (result of B7 or C7 or D7)
Sheet 2, Cell B7 (variable)
Sheet 3, Cell C7 (variable)
Sheet 4, Cell D7 (variable)

Problem to solve:
If Sheet 2 is active, value of cell A7 is equal to cell B7, if sheet 3 is active, value of cell A7 is equal to cell C7 and if sheet 4 is active, value of cell A7 is equal to cell D7.

Example data:
Sheet 2, Cell B7 = 100
Sheet 3, Cell C7 = 200
Sheet 4, Cell D7 = 300

Result:
If sheet 2 is active value of cell A7 =100
If sheet 3 is active value of cell A7 =200
If sheet 4 is active value of cell A7 =300

Thank you in advance…