The linear momentum of a $3 \mathrm{MeV}$ photon is:

Energy of a photon, $E= 3 \mathrm{MeV} = 3 \times {10}^6 \mathrm{eV}$

Linear momentum of the photon,  $P =\frac{E}{c}$.

where $c$ is the speed of light in the vacuum.

$P = \frac{3 \times {10}^6 \mathrm{eV}}{3 \times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}} = {10}^{-2} {\mathrm{eVsm}}^{-1} = 0.01 {\mathrm{eVsm}}^{-1}$