You need to assign numbers that you have left out,
Let's say SUM of the numbers are 50
The 3rd number is 2 times the 2nd.
3rd number 40 less than the 2nd.
Let's assign ( X , Y , z ) to 3 numbers in order. Now we convert English statements, to their algebraic
X + Y + Z = 50
Z = 2Y
Z = Y -40
Now we have 3 equations and 3 unknowns to solve.
We observe that 2nd and 3rd equations both are equal to Z
2Y = Y - 40
Y = -40
Z = -80 / substitution in 2nd equation
X - 40 - 80 = 50
X = 50 + 120 = 170.
See, this is not the way to learn, where you don't even give enough information to have an answer.