The order of bond angles in NH3, PH3, and AsH3 is:

The electronegativity order of N, P, and As is N > P > As. So bonding electron pairs will be nearest to N nucleus in NH3 in comparison to other molecules and so will be affected less by the lone pair of electrons of N. The bonds will bend comparatively less in NH3. Thus, if the electronegativity of the central atom increases the bond angle also increases.