In CH4, there is no lone pair around the central atom carbon and hence, the bond angle is normal i.e. 109.28'. But in NH3, one lone pair is there around the central atom nitrogen and due to more lp-bp repulsion than bp-bp, bond angle decreases to 107degrees. Similarly, in H20, there are two lone pairs around the central atom oxygen and hence lp-lp>lp-bp>bp-bp and hence bond angle decrease to 104.28'. hence, ur desired result
H20 < NH3 < CH4.
Thanks.