I know this question has likely been asked before, but I am horribly confused and need some help with this.

Let's say we have a system whose initial state at t = 0 is given in terms of a complete and orthonormal eigenvector of the Hamiltonian:

$$ | \Psi(0)\rangle = \frac{1}{\sqrt{3}} |\phi_1\rangle+ \frac{1}{\sqrt{2}} |\phi_2\rangle+ \frac{1}{\sqrt{6}} |\phi_3\rangle$$

How would you find the probability of finding the system, at a time t, in the state $|\phi_3\rangle$?