I feel like the answer is

126
Square each value, add together, divide by numbers of values (get the average/mean)

But the third clue seems to prohibit addition, so I'm not sure.

The answer is

126

Square each value, add together, divide by numbers of values

(3² + 12² + 15²) / 3 = (9 + 144 + 225) / 3 = (378) / 3 = 126

But the third clue seems to prohibit addition, so I'm not sure.
The third clue was changed to only prohibit subtraction.

Full list of equations:

(12 + 32) / 2 = 5

(12 + 42 + 52) / 3 = 14

(22 + 42 + 62 + 72 + 102) / 5 = 41

(72 + 132) / 2 = 109

(22 + 22 + 42) / 3 = 8

(12 + 42) / 2 = 8.5

(42 + 52) / 2 = 20.5

(22 + 42) / 2 = 10

(22 + 22) / 2 = 4