=SUMPRODUCT(SUMIF(INDIRECT("'"& {"sheet1","sheet2","Sheet3"}
&"'!A:A"),C2,INDIRECT("'"& {"sheet1","sheet2","Sheet3"} &"'!B:B")))
The above formula can further be shortened..
if the sheetnames are named in a sequencial way.
Sheet1, Sheet2, Sheet3 fall into the sequential naming convention.
=SUMPRODUCT(SUMIF(INDIRECT("'"&"sheet"&{1,2,3}&"'!A:A"),C2,INDIRECT("'"&"sheet"&{1,2,3}&"'!B:B")))
If you had 100 sheets you wouldn't want to do this:
{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15....100}
=SUMPRODUCT(SUMIF(INDIRECT("'"&"sheet"&ROW(INDIRECT("1:100"))
&"'!A:A"),C2,INDIRECT("'"&"sheet"&ROW(INDIRECT("1:100"))&"'!B:B")))