A body of mass m is placed on earth surface which is taken from earth surface to a height of h = 3R then change in gravitational potential energy is : -

Gravitational potential energy at distance r from the earth's centre: $U = -\dfrac{GMm}{r}$
Initial (on the surface, r = R): $U_i = -\dfrac{GMm}{R}$
Final (h = 3R, so r = R + 3R = 4R): $U_f = -\dfrac{GMm}{4R}$
Change in GPE $= U_f - U_i = -\dfrac{GMm}{4R} + \dfrac{GMm}{R} = \dfrac{GMm}{R}\left(1 - \dfrac{1}{4}\right)$
$= \dfrac{3}{4}\dfrac{GMm}{R} = \dfrac{3}{4}\dfrac{GM}{R^2}mR = \dfrac{3}{4}mgR$ (since $g = \dfrac{GM}{R^2}$)
(The formula mgh cannot be used, because h is not small compared with R.)