3a. It subtracts pairs of numbers, to a total of [N] pairs if [N] is positive, or one pair otherwise. The first member of each pair is found in a list starting at location 476, counting down; the other member of the pair is found in a list starting at location 600, counting up. The sums are placed in a list starting at location 700, counting up.

3b.
2000012701MOV #500, R1
2002000500
2004012702MOV #600, R2
2006000600
2010012703MOV #700, R3
2012000700
2014016704MOV 2042, R4
2016000022
2020004767JSR R7, 2026
2022000002
2024000000HALT
2026014100MOV -(R1), R0
2030162200SUB (R2)+, R0
2032010023MOV R0, (R3)+
2034005304DEC R4
2036003373BGT 2026
2040000207RTS R7
2042000040.WORD 40

[on to problem #4]