1. 'The one with 3D screens' is Sense 3.0 - it has nothing to do with any of the Android operating systems, except that it needs Gingerbread 2.3.5 to work. Sense is HTC's graphic user interface that works on top of Android.
Honeycomb is Android version 3.0 and is designed for tablets, not phones.
2. HTC have announced that Ice Cream Sandwich will come to the Desire S. However, there's a big question as to when it will get to your phone as it will depend on whether or not you have a branded phone. T-Mobile seems to be the slowest to get around to releasing updated ROMS for its brand, so you might be in for a very long wait, unless they're going to skip the Sense 3.0/2.3.5 update (which they seem to have pulled?) and go straight to ICS. The non-branded HTC ROMs will get ICS first.
3. Yes, it toggles between the two.