Determine valence electrons.

Hydrogen (H) has 1 valence electron. Oxygen (O) has 6 valence electrons.

Oxygen is the central atom. Each hydrogen atom shares one electron with the oxygen atom, and the oxygen atom shares one electron with each hydrogen atom, forming two single covalent bonds. The oxygen atom also has two lone pairs of electrons.
Using $\times$ for hydrogen's electrons and $\cdot$ for oxygen's electrons:

The bonding in water ($\text{H}_2\text{O}$) is covalent bonding. This occurs when atoms share electrons to achieve a stable electron configuration.

Nitrogen (N) has 5 valence electrons. Hydrogen (H) has 1 valence electron.

Nitrogen is the central atom. Each hydrogen atom shares one electron with the nitrogen atom, and the nitrogen atom shares one electron with each hydrogen atom, forming three single covalent bonds. The nitrogen atom also has one lone pair of electrons.
Using $\times$ for hydrogen's electrons and $\cdot$ for nitrogen's electrons:

The bonding in ammonia ($\text{NH}_3$) is covalent bonding. This involves the sharing of electrons between nitrogen and hydrogen atoms.

Aluminum (Al) is a metal with 3 valence electrons.

For metals like aluminum, a dot and cross diagram in the traditional sense (showing discrete shared or transferred electrons between two atoms) is not applicable. Instead, metallic bonding is described. Aluminum atoms lose their valence electrons to form positive ions ($\text{Al}^{3+}$). These valence electrons become delocalized and form a "sea" of electrons that are shared among all the positive metal ions in the lattice.

The bonding in aluminum is metallic bonding. This involves a lattice of positive metal ions held together by a "sea" of delocalized valence electrons.