How to put a hole in a 3" glass float? Answered

you need a special drill bit like this and drill at low speed withou much strenght, otherwise the glass can crack and broke. Take as long as you need.
In my classroom two pals drilled glass. One toke 1 hour to drill a bottle. The hole got smooth. The other toke 5 minutes. The bottle got cracke and the hole square. So, take it slow.

http://www.homedepot.ca/product/1-2-inch-glass-tile-bit/969018

I've used a product like this (from any home improvement store) with GREAT ease to cut various holes in glass. Cuts the glass like butter, and leaves a nice straight edge. A little wet sandpaper (while wearing gloves) takes care of any sharp edges.

The exact ones I used looked more like
http://image.made-in-china.com/2f0j00MvFafuRCgYbB/Glass-Tile-Drill-Bit-TWD00023-.jpg

Which ever method you use you should wear gloves and EYE PROTECTION. These glass floats already have odd stresses in them and when cut may release those stresses and break throwing off chips. Cutting under water as mentioned by Burf will help greatly.

Thanks all. I'll report back and let you know how it turns out.

Use a 1/2" diamond plated hole saw. It will cost you $15 to $20 U.S. in most hardware stores.
Chuck the bit into a BATTERY POWERED drill and hold the glass under a slow but steady stream of water and slowly drill the hole where needed. Don't apply any more pressure to the drill bit than needed to lightly abrade away the glass. The break-through point, where the bit finally cuts through the glass, is the most precarious, where most breakage occurs.
Practice a few times on some old bottles to get a feel for the process. And, expect to break a few no matter how carefully you work.

You can buy hole cutting saw blades for glass. Just pick the right diameter and steady the glass float in a padded vice (just enough that it won't move around), and carefully make your hole.

I've seen holes cut into bottles this way (which worked fine), but I don't think there's any process that can ensure you won't break a few of the floats.