Sometimes things are mundane, sometimes they are magickal signs, and sometimes they might be a mix of both- the reflection may be caused by the light through the window, but if you felt it had power and a message for you, then I would warmly say to trust your intuition! 
Three is a very powerful number in magick and spirituality. From symbols of triplicity to gatherings of three, it is a number with many possible meanings:
In this case, since it appeared as three moons, it may be a symbol of a Triple Moon Deity. There are few Goddesses associated with triple moons, the first two coming to my mind are the Triple Goddess and Hekate
Perhaps they could be worth looking into to see if there is a connection? I would urge you to keep a sharp eye out for any other potential signs and make note- keeping a list in journal is a good way to keep track! 
Blessed be, @BlueAngelite! 