What is the number of integer solutions to x1+x2+x3=12x_1 + x_2 + x_3 = 12x1+x2+x3=12 such that x1≥1,x2≥2,x3≥3x_1 \geq 1, x_2 \geq 2, x_3 \geq 3x1≥1,x2≥2,x3≥3?
15
21
28
36