Is it possible to join workbooks?

I have 3 employees that are using one sheet for each event that they budget
for. Each employee has a different workbook because they have so many
events. Within each employees workbook I have used sheet one to total all
the following sheets to the appropriate budget line. So know I have 3
workbooks with sheet 1 that need to be added togeather to total the budget
for the whole department.