The number of electrons in 3p orbital of trivalent phosphorous is

Atomic number of P is 15 and its electronic configuration is $1{\mathrm{s}}^2 2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^3$.

When it becomes trivalent then its electronic configuration becomes:

${\mathrm{P}}^{+3}-1{\mathrm{s}}^2 2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^0$