Postulate 3

The only possible result of the measurement of a physical quantity A is one of the eigenvalues of the corresponding observable $\hat{A}$.

This means that each "individual measurement" returns only one of the eigenvalues of $\hat{A}$. Recall that eigenvalues are defined by:

$$\hat{A}\ket{\psi} = \ket{\psi} a,$$

where $\hat{A}$ is an observable (operator), $\ket{\psi}$ is a vector denoting the state of the system, and $a$ is the quantity measured.