splunk eval row with last field

Hello Splunkers i requiered eval the last field with current row. example: field 1 ...... field2.........field3........................................................................result 1..................1..............(field1+field2)...........................................................field3 3..................4..............(last_field3 + current field1) - current field2) ... current field3 7..................2..............(last_field3 + current field1) - current field2) ... current field3 numeric example field 1 ...... field2.........field3...........................result 1..................1..............(1+1)...............................2 3..................4..............(2 + 3) - 4) ..................... 1 7..................2..............(1 + 7) - 2) ......................6 thanks!!!