Is it true that it only takes three days?

PADI courses are "performance based," which means that you only earn your scuba certification when you demonstrate that you have mastered the required skills and knowledge.  Some people learn faster than others, so how long it takes you may vary. 

The PADI Open Water Diver course (beginning scuba) is typically split into five or six sessions with tremendous flexibility. The course may be scheduled over as little as three or four days, or as much as five or six weeks, or something in between depending upon student needs and logistics.   It is rare but you could complete your PADI scuba certification in as little as 3 days.