>>...The sum of three numbers is 72...<<
Smallest + Middle + Largest = 72
>>...The middle number is one less than three times the smallest number...<<
Middle = 3 times Smallest minus 1.
>>...The sum of the smallest and the middle number plus ten equals the largest number...<<
Smallest + Middle + 10 = Largest
So you have this system:
Can you solve that system? If not post again asking how.
Answer: S = smallest = 8, M = middle = 23, L = largest = 41
Edwin