There is nothing unusual or wrong about "Il est quinze heures." The French often use 24-hour time so this is simply "It is three p.m." It's not just military time and it's not a challenging mathematical problem. Take it as an opportunity to learn something about the culture surrounding the language you are learning.
that would be 3:00 hours, or 3:00 a.m. you'd have to write 3:00 p.m. to get it right in 12-hour time. 24-hour time is much easier; you don't get morning and afternoon mixed up. As an amateur astronomer, I've used 24-hour time for about 40 years, and now I'm finding myself less willing to translate to olde-fashioned time for my work colleagues. More so, since we all use 24-hour time on the time sheets at work.