Why can't you calculate this?
P=1500x-(100+400x)
P=[1500 x 3] - [100 + (400 x 3)]
P=4,500 - [100+1,200]
P=4,500 - 1,300
P=3,200 - Profit for 3 roofs
$3,200 / 3 =$1,067 - average profit per roof.
He will ALWAYS make a profit, because his profit margin is 1,500 per roof, while his costs are fixed at $400 per roof plus a small amount of $100.