Since one out of every 3 rows will contain tomato plants, there will be 15 / 3 = <<15/3=5>>5 rows of tomato plants.
This gives her enough room to plant 5 rows * 8 plants/row = <<5*8=40>>40 tomato plants.
The plants will produce a total of 40 * 3 = <<40*3=120>>120 tomatoes.
#### 120