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I have come up with my own theory to work out the three neighbours. My theory is that all the differences of all the numbers added together is the difference between the two whole numbers.

1+2+3=6
2+3+4=9
The difference between 1 and 2 is 1.
The difference between 2 and 3 is 1.
The difference between 3 and 4 is 1.
1+1+1=3. The difference between the whole numbers is .
All the differences added together is 3.

45+46+47=138
21+22+23= 66
The difference between 45 and 21 is 24.
The difference between 46 and 22 is 24.
The difference between 47 and 23 is 24.
24 x 3=72 / 138 - 66= 72
The difference between the three smaller numbers and the difference between the one large whole number are both 72.