Here's a breakdown of what you may be thinking of:
* Gohan, Goten, and Trunks vs. Piccolo: This happens in the Buu Saga, specifically during the World Tournament. Piccolo trains Goten and Trunks for the tournament, but Gohan is already a formidable fighter at this point. While there are some training scenes with Piccolo, Gohan, Goten, and Trunks, there isn't a direct fight between them in the tournament.
* Goku vs. Piccolo: This happens in the Saiyan Saga and in the Namek Saga. While their initial fight is early in Dragon Ball Z, it is not a fight with Gohan, Goten, or Trunks. Goku and Piccolo do team up in the Namek Saga to fight against Frieza.
* Goku, Gohan, Goten, and Trunks vs. Buu: This happens in the Buu Saga, during the final fight against Buu. They team up to fight against Buu's various forms.
It's important to specify what you are looking for to provide you with a more accurate answer.