A lot of people are suggesting "good lighting", which is a good idea, of course.
But there're many different types of lighting, big strobes, hot monolights, flashes/speedlites.
Seeing as you're going to be photographing Ice Cream, you'll want to steer away from monolights, you want photo of Ice Cream, not Puddle of Cream.
So I'd be going for flashes. 2 or 3 430EX should be sufficient for good lighting, search ebay for 'softbox' and you'll get a cheap plastic thing for $5 that sits over the flash head. Best $5 I ever spent.
But as you've gone for a 5D3, you'll need a way to trigger them, so you'll need an ST or 580EX (i'd suggest a 580EX on-camera or on a cord as master, and the 2-3 430EX on wireless slave). That's certainly a bit of overkill if that's all you're ever going to photograph, so maybe consider renting 1 or 2 for the weekend.
Or someone previous mentioned YongNuo and off/brand triggers, i'll presume they're as good but i've never used them.
After that, the lens is secondary. The 100LIS macro is good, very good, especially with the IS. But the 100 USM non-L macro is just as sharp, half the price, and you won't miss the IS on a tripod. If you're going to use it for other things afterwards then maybe the 100L makes more sense. Also consider the 100 f/2.0, I've taken great macros with that on Kenko Extension Tubes (there's a Lens Gallery around here somewhere).
Also consider that for Ice Cream you won't need 1x macro anyway, including the cone or bowl you can get away with 0.25x macro which most lenses will do. The 24-105L you got with the camera does 0.23x macro anyway, so if I were you I'd just experiment with that and maybe cropping 5% off the edges will be enough. Later on, if you want to print billboard-sized photos for the side of your Ice Cream Van or shop, then consider a dedicated macro lens.