I'm sure you could look up how many seconds are in three days, but to do it by yourself with math:
There are 60 seconds per minute, and 60 minutes per hour. 60*60=3,600. There are 3,600 seconds per hour. There are 24 hours per day. 3,600*24=86,400. There are 86,400 seconds per day. 86,400*3=259,200. There are 259,200 seconds in 3 days.
I hope this helps :)