Answer from cs61c-eb (Dae-Yeon Kim 16568138) for Question 2
The three come from the result of ALU, the result after
accessing the value in the memory, or the value from the register.

1. when we need the result of ALU
 add $t0, $t1, $t1
 add $t0, $t0, $t0

2. when we need the result after accessing the memory
  lw $t0, 0($sp)
  add $t1, $t0, $t0

3. when we need the value from the register
  add $t0, $t1, $t1
  add $t2, $t3, $t3