Question 941110
Let the ONE no. be y, the first no. be x and the third no. z
From the statement
y = 3 * x ----eqn1
z = x + 100 -----eqn 2
x + y + z = 405 -----eqn3

let's rearrange the eqns and sole the 3 simultaneous eqns.
3*x - y = 0
x - z = -100
x+y+z = 405

all the best