First, it is a concept of VSEPR (Valence Shell Electron Pair Repulsion Theory) .
Hydronium ion is
So, here the central atom is Oxygen
Since, there is
So, In total, there are
So, it's distribution is TETRAHEDRAL , and the shape is Trigonal PYRAMIDAL in order to allow least possible repulsions.
So, lewis structure and shape of Hydronium ion is :
Hope it Helps :)