Let the first number be x.
Then the second number is 3x.
Then the third number is (x+100).
Sum of the numbers=500
x+3x+(x+100)=500
4x+(x+100)-500=0
5x=400
x=80
The first number is 80.
The second number is 240.
The third number is 180.
We can check by adding all three of those numbers together: 80+240+180 = 500
Hope this helps! If you have any questions just ask!