Convert Three-Way to Single Switch and Add Smart Switch
You don't have the neutral you think you do

This particular light was wired as a spurred three-way switch off of an old-style switch loop, so you actually do not have a neutral at either switch box, despite the white wires being present in both.  In your case, we can tell white is the always-hot wire at both switch locations, even, from the fact that it's the white wire nutted to the bundle of black wires in the light box.

From there, we know that the black wire in the cable from the light to switch A is the switched-hot, and that the black and red wires in the cable between switch A and switch B were the travellers in your old setup.  Since there's no neutral in all this, and the Insteon 2477D requires a neutral to work (i.e. power the smarts inside), we can't use that part here by itself.

Never fear, though! Insteon to the rescue!

However, since you are working in the wonderfully flexible system that is Insteon, there is a way to get what you want, simply by adding some more parts. You will need an Insteon In-lineLinc&trade; Dimmer module (model 2475DA1) for this, in addition to the dimmers you already have.

Once you have the correct pieces for this job, you'll need to rewire things as follows, starting with the fixture box:


All the fixture-box blacks get nutted to each other and to the in-line dimmer's black (LINE) wire (but not the fixture's black wire! otherwise, the light will simply be on all the time)
All the fixture-box whites get nutted to each other, the in-line dimmer's white (NEUTRAL) wire, and the fixture's neutral wire
The bare (GROUND) wire from the in-line dimmer gets nutted to the existing bare pigtail from the box and the fixture's bare ground wire
The fixture's black wire, finally, gets nutted to the red (LOAD) wire from the in-line dimmer


At this point, we move on to the first switch box, where the dimmer that replaces switch A needs to be wired as a 3-way secondary controller:


The two black wires that went to switch A get nutted to each other and to the black (LINE) wire on the first dimmer
The two white wires associated with the switch A circuit that are nutted to each other need to have the white (NEUTRAL) wire from the first dimmer nutted in with them
The bare ground wire that went to switch A needs to be nutted to the bare (GROUND) wire from the first dimmer
The red wire that went to switch A and the red (LOAD) wire on the first dimmer both need to be capped off individually, as neither wire is used in this configuration


Now that we have switch A done, we can button up that switch box and move onto wiring switch B as an Insteon 3-way secondary as well:


The black wire that went to switch B gets nutted to the black (LINE) wire on the second dimmer
The white wire that went to switch B gets nutted to the white (NEUTRAL) wire on the second dimmer
The bare ground wire that went to switch B gets nutted to the bare (GROUND) wire on the second dimmer
And, finally, the red wire that went to switch B and the red (LOAD) wire on the second dimmer both need to be capped off individually, as again, neither wire is used in this configuration.


Now, you can button switch B's box up, turn the breaker on, and set to work configuring the Insteon links so that the two wall dimmers are linked to each other (in a 3-way cross-link) and to the in-line dimmer module (so that it can receive commands from both dimmers).  Once the Insteon links are configured, you can turn the breaker off again, put the ceiling fixture back up, turn the breaker on, and enjoy your multi-way dimming setup!