After a disastrous third quarter that left it flat for the year, hedge fund mogul Bill Ackman is coming back. Ackman’s $11.5 billion Pershing Square hedge funds gained about 8 percent in October, according to a note sent to investors, a copy of which was reviewed by The Post. The funds are up about 8.5 percent for the year. All of Ackman’s holdings gained for the month, but the standout was Canadian Pacific, which rose 16 percent. That stock has tripled since Ackman bought it, leading him to sell about 30 percent of his stake last month for $835 million. It remains one of his biggest holdings.