CH4 , H2O , NH3 order of increasing bond angle around central atom
You are right they are all tetrahedral if you consider all the electron pairs around the central atom. however only CH4 is truly tetrahedral as it has 4 equivalent bonding pairs of electrons so making expected bond angles for a tetrahedron of 109o or so.
Nitrogen has one lone pair of electrons this pair has a slight distorting effect on the other 3 bonding pairs, squeezing them together giving a bond angle smaller than 109 (107) giving a trigonal pyramid shape wrt the bonding pairs alone.
Oxygen has two lone pairs in water, squeezing the two bonding pairs further, giving a bond angle of 104.5, the molecule is described as bent wrt the bonding pairs.
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.