Question

is neutral pion is equal to triple gamma?
is allowed or forbidden? if forbidden give reason

Answer 1

Indeed, neutral pions usually decay into gamma rays according to the following reaction:

$$\pi^0\rightarrow2\gamma.$$

The decay into three "gamma photons" is forbidden by the C-symmetry: The intrinsic C-parity of a neutral pion is +1, the C-parity of a system that consists of n photons is (-1)ⁿ, which means that odd number of photons results in asymmetry.