The valency of an atom with three protons and four neutrons is:

The mass number of atoms equals the total number of protons plus neutrons.
As a result, mass number = 3 + 4 = 7.
Now, the valency of an atom equals the mass number of that atom.
Atom valency = 8 - 7 = 1.
As a result, the compound requires one electron to be stable.
Furthermore, because the compound has three protons, its electronic configuration will be 1s^2,2s¹.
The molecule will lose one electron to achieve a stable electronic configuration.
As a result, its valency is 1.