Each son receives 300 / 3 = <<300/3=100>>100 pieces.
Rhys places 25 x 2 = <<25*2=50>>50 pieces.
Rory places 25 x 3 = <<25*3=75>>75 pieces.
All three boys place 25 + 50 + 75 = <<25+50+75=150>>150 pieces.
The boys have not placed 300 - 150 = <<300-150=150>>150 pieces.
#### 150