Question

which valency should be use in Phosphorus for its compound formation while it has two valencies 3 and 5?

Answer 1

you can use either 3 or 5 because if the valency is more than 4, then 8 should be minus therefore 8-5 = 3.

Answer 2

Normally Phosphorus forms very stable compounds and ions with its valency +5.  Compounds with P in valency 3 are also formed. But less stable compared to the others with valency 5.

Phosphorus has a configuration of electrons as: 3s2 3p3. So initially the 3 electrons from 3p are shared or lost. But when there is more ionization energy is available or when there is a good electronegative element/radical is near Phosphorus, the 2 electrons in 3s orbital are also lost. 

Many salts of P are formed with P in +5 valence state.  Phosphorous acid is formed with a valence state of +3.