You have likely seen a group of three periods together, like this: … This is actually treated as just one punctuation mark, and it is called an ellipsis (plural ellipses). Ellipses are used when a thought trails off, or when a particularly long pause occurs. "It can't be…." "I think… he may have betrayed us."
Ellipses are also used in quoted material to indicate omission. A quote like "This book is unlike anything I've ever seen. It's poignant and profound." could become "This book is… poignant and profound."
When a sentence ends with an ellipsis, you still need to put a period at the end of the sentence, which will mean there are four dots in a row. "It can't be…."
Be careful with ellipses — when used unnecessarily, they can make your writing seem sloppy or confuse the reader. However, when used properly, they can make dialogue seem more realistic and convey tone well.