Simplify the following text for a Grade 8 native English-speaking person.
Original: The work took three years , and Nicola was helped by his son Giovanni Pisano and his assistants Arnolfo di Cambio , Lapo di Ricevuto and several other artists .
Simplified: He was aided by several assistants , among which were Arnolfo di Cambio and Lapo di Ricevuto .