Answer
Hydronium ion (H3O+) forms when there is an interaction between a H2O molecule and a proton. So when this interaction occurs, pure water dissociates into a hydronium ion (H3O+) and OH- ions, this happens at an equilibrium of these ions (at a constant rate). pH is the negative log of the H3O- ion ( pH = -log [H3O-]). So that is the relationship between pH and H3O+.
When the pH is 7, it indicates neutral solution where hydronium and hydroxide ions are at equilibrium, so when you add an acid the solution increase the concentration of protons and lowers pH value indicating an acidic solution. Diluting an acid results in the reduction of the hydronium ion concentration and raises the pH.
When you add a base, the solution increases the concentration of hydroxide ions that neutralise hydronium ion increasing pH. So when you dilute a base, this leads to the reduction of the concentration of hydroxide ions, thus increasing the concentration of hydronium ions and lowering pH.