I am doing a 5 tier wedding cake and will be putting 3 rows of fondant ruffles down the side of this cake. Anyone have any suggestions on keeping the ruffles in place?

Water (if adhering to a fondant-covered cake).

A butter cream ruffle would be the way I'd do it using a ruffle tip is pretty easy. Im not sure about a fondant ruffle maybe some kind of pick hidden to hold the weight.

Use gum glue. Just add water to gumpaste and let it dissolve to form a glue. It holds fondant to fondant very well. If you are worried about the weight of the ruffles, make them in advance and let them dry.
They will be a lot lighter and easier to attach. The gum glue will hold wet to dry fondant, no problem.

Your topic says ruffles on buttercream ... so when attaching fondant to buttercream I use some piping gel just brush a small amount on the fondant & it will attach to the buttercream just fine.

If you decide to do the ruffles with buttercream wilton tip 127 to do basic ruffles or wilton tip 86 does a really nice ruffle boarder. I use tip 86 most

Hope this helps