The Gathering question

I have read all 3 of the darkest powers books and enjoyed them all.,I don't think you need to read the short stories first,I'd say that if you have a copy of the Rising there you could go ahead and read it, just read the short stories later.

Two of the short stories are the same events that happen in the Darkest Powers trilogy, except from a different character. One is a prequel about Derek's life. The other two are after the series ends and do provide a little more background on the Darkest Powers' situation, but are not brought up in The Rising. The only thing that you learn in the short stories before The Rising is the gang's location, which is not necessary to know prior to reading The Rising. :)
I would recommend reading the Darkest Powers trilogy if you have not yet done so! Will it affect your reading of The Rising? Nope, but reading The Rising first may spoil some aspects of the Darkest Powers trilogy if you decide you want to read them later!