One number Y is 3 times a first number X. 
Y = 3X
A third number Z is 100 more than the first number X.
Z = X + 100 
The sum of the three numbers is 585
X + Y + Z = 585
Can you solve this system of equation:
ìY = 3X
íZ = X + 100
îX + Y + Z = 585
If not post again asking how.  The solution is
X = 97, Y = 291, Z = 197
Checking in the words, not the equations:
291 is 3 times 97.  197 is 100 more than the 97. 
The sum of 97, 291 and 197 is 585.  That checks.
Edwin