what was the purpose of the three friends assembling again?

Do you mean at the beginning of the story? The narrator, J., is smoking in his room with his friends, George and William Samuel Harris, and his dog Montmorency. The men, all hypochondriacs, are chatting about their latest illnesses, each man certain that he is in danger of death or serious disease.

the three friends were assembling again because they wanted to discuss and arrange the plans they made and the things they should take with them when going for the journey.