Assuming acid strength relates directly to the electronegativity of the central atom, rank ${\mathbf{H}}_{\mathbf{3}}{\mathbf{PO}}_{\mathbf{4}}$, ${\mathbf{HNO}}_{\mathbf{3}}$, and${\mathbf{H}}_{\mathbf{3}}{\mathbf{AsO}}_{\mathbf{4}}$ in order of increasing acid strength.

Electronegativity of an atom is defined as the tendency of an atom to attack the electrons.

In the periodic table as we move from right to left in a period electronegativity goes on increasing. While in a group as we move downward electronegativity is decreases. This is the general trend found in the periodic table. The relation of acidic character with electronegativity is as follows;

Electronegativity $\alpha$ acidic character

The relation of acidic character with electronegativity is as follows;

Electronegativity$\alpha$acidic character

In a group, as we move downward electronegativity is decreases.So nitrogen is more negative than phosphorus and is more electronegative than arsenic. Hence the order of acidity for the given acids is as follows;

${\mathrm{HNO}}_3>{\mathrm{H}}_3{\mathrm{PO}}_4>{\mathrm{H}}_3{\mathrm{AsO}}_4$

So nitric acid is most acidic one.