Answer:
The reason why phosphorus usually forms 3 bonds is because of its valence electrons. Phosphorus has 5 valence electrons and due to the octet rule, it will strive to gain 3 more in order to fill it outer shell and become stable. PH3 and PCI3 are great examples of this. In the compound PH₃, The phosphorus atom is in a covalent bond to three hydrogen atoms. Each of the three hydrogen atoms share their one valence electrons with phosphorus, and the phosphorus atoms shares one electron with each hydrogen atom. In the compound PCl₃, one phosphorus atom shares one electron with each chlorine atom, and each chlorine atom shares one electron with phosphorus. Note that each of these compounds are covalent bonds. As show, phosphorus forms 3 bonds because it needs 3 more electrons to become stable.
