O plural de he, she e  it é they. Assim o adjetivo possessivo para he é his, para she é her e para it é its, enquanto que o plural do adjetivo possessivo da terceira pessoa do singular e da terceira do plural é their.

He and his brother have a large company. Their company is very big.

She and her sister teach at the same school. Their school is near their house.

My two dogs need a new house. Their house is too old.

Where are my new magazines? They are on the kitchen table.