Erratically fast moving white star like craft, dimmed down and vanished

Went out on a late night walk, went to a hill overlooking downtown to admire the city and (mostly) the super moon. It’s hard to see stars because we’re in the inner city, and only handful standout. There were 3 stars in a triangle formation, and suddenly one broke off. It was moving very fast and flying irrationally moving in all directions,  before stabilizing and flying horizontally. It was originally a very bright white and seemed to dim down before all of a sudden it flashed to black and vanished.