The three of us sisters started a coffee shop together.
Literal
We [topic-は] sisters 3-people [with-で] coffee-shop [object-を] started [polite-past-めた].
The で here is the 'in a group of' marker — used with numbers and group descriptions to mean 'as a group of X.' 3人で ('in a group of 3') modifies the subject 私たち. 姉妹 ('sisters') + 3人 clarifies the group composition. Common in warmhearted personal anecdotes.