Good for you Brian, this is a great entry into a challenging genre. Having done this myself, you tend to need a longer lens in most circumstances.
Depending on trhe venue for the ceremony your 70-300L could be your best option, so you don't have to zoom with your feet. If you are not the primary photographer you are naturally granted less access and your zoom will be handy to get you close to the action, depending on lighting.
If you are familiar with the venue, your 85mm of 100mmL may be a better option if you are introduced as the "second" photographer and are allowed to walk up and down the aisle and get in close to the action.
The post ceremony "family" shots are the hardest if you are not the primary photographer as the subjects will give their attention to the primary photographer and it doesn't matter how hard you try your shots will be as good as the people standing beside you with a point and shoot.
Your 15-85 will come in handy, as the cocktail party shots tend to be very cramped and framing can be difficult.
I have had a great time when I have done this and I am sure you will too.
Have fun and good luck.