What you've written in your
crontab entry doesn't match your stated need. The manpage (
man 5 crontab) is reasonably clear on this:
Step values can be used in conjunction with ranges. Following a range with "/" specifies skips of the number's value through the range. For example, "0-23/2" can be used in the hours field to specify command execution every other hour [...]
To run every 15 minutes starting at 5 minutes past the hour, I'd go for the simple
5,20,35,50. A step value such as
5/3 should also work, but in this instance I'd suggest that its intent is less obvious.