Let's sum these two numbers.
3 + 15 = 18 total hours.
In every 12-hour the clock format changes AM to PM or the opposite PM to AM. So we need to divide 12 with the result 18.
There are 1 times 12 number in the 18
If the result is ODD number the time format will change the opposite side. If the result is EVEN number the time format will be the same.
What is 15 hours from 3pm ? = 6am.