To find the number of molecules with non-zero net dipole moment, we must check:

• whether the individual bonds are polar, and
• whether the molecular geometry causes the bond dipoles to cancel.

Now examine each molecule:

(i) $BeF_2$

• Shape: linear
• Two $Be-F$ bonds are polar, but they are equal and opposite.
• Hence dipole moments cancel.
• Net dipole moment $=0$

(ii) $BF_3$

• Shape: trigonal planar
• Three $B-F$ bonds are polar, but arranged symmetrically at $120^\circ$.
• Their vector sum is zero.
• Net dipole moment $=0$

(iii) $H_2O$

• Shape: bent (due to two lone pairs on O)
• $O-H$ bonds are polar.
• Because of bent geometry, dipoles do not cancel.
• Net dipole moment $\neq 0$

(iv) $NH_3$

• Shape: trigonal pyramidal (one lone pair on N)
• $N-H$ bonds are polar.
• Due to pyramidal shape, dipoles do not cancel.
• Net dipole moment $\neq 0$

(v) $CCl_4$

• Shape: tetrahedral
• Four $C-Cl$ bonds are polar, but arranged symmetrically.
• Dipoles cancel.
• Net dipole moment $=0$

(vi) $HCl$

• Diatomic molecule
• $H-Cl$ bond is polar.
• In a diatomic molecule, net dipole is just the bond dipole.
• Net dipole moment $\neq 0$

Molecules with non-zero dipole moment are: $H_2O,\ NH_3,\ HCl$

Therefore, the total number is: $3$