To solve for X in the first expression:

X + 7 = 12
Subtract 7 from both sides:
X = 5

To solve for X in the second expression:

3 = 4(x-1)
3 = 4x - 4
Add 4 to both sides:
7 = 4x
Divide by 4 on both sides:
x = 7/4

To solve for X in the third expression:

x-1 = 6
Add 1 to both sides:
x = 7

Therefore, X is equal to 5, 7/4, and 7 for the three given expressions.