Because if the formula is P= a+b+c and you triple each side you get P= 3a+3b+3c so it would be three times as big as the old perimeter.
For example if your equation is P=3+2+5 (which equals 10) and then you multiply each side by 3 your new formula is P= (3•3) (3•2) (3•5) which equals 30. 30 is three times larger than 10.
