Complex sentence: Should a city already have three cubes and a new cube is to be added, an Outbreak occurs and each interconnected city gains one more cube of that color; this can create a chain reaction across many cities if several already have three disease cubes on them.
Split into simpler sentences: Should a city already have three cubes and a new cube is to be added, an Outbreak occurs, and each interconnected city gains one cube of that colour. This can create a chain reaction across many cities if several already have three disease cubes on them.