Problem:

Counting the total number of values from the range A1:B3 on Sheets 1, 2 & 3 that are greater than the criteria specified in cell A2.

Solution:

Use the SUM, COUNTIF, and INDIRECT functions as shown in the following formula:
=SUM(COUNTIF(INDIRECT(\"Sheet\"&{1,2,3}&\"!A1:B3\"),\">\"&A2))