Call the three integers you're looking for A, B and C, with A<B<C.

To get A, just divide the first number in the input list by 3. It has to be A+A+A
To get B, take the second number in the list and subtract 2A. It has to be A+A+B
To get C, divide the last number in the list by 3. It has to be C+C+C

Here is some python code:

input()
observations = [int(x.strip()) for x in input().strip().split()]
a = observations[0]//3
b = observations[1] - a - a
c = observations[-1]//3
print( a, b, c )