3 DI vessels? I dunno, lets play around with some figures but I doubt there would be any advantage over two.
(all totally theoretical so do not take them at face value)
The way 2 DI vessels work is this, imagine you have a red-DI and a green-DI (yes I know they are usually blue but it just makes it easier to visualise). If you put the red one first and the green one second it will do this...
Lets say RO water = 20ppm
1 liter of resin in red-DI will last 1000 liters before it starts to rise
1 liter of resin in green-DI won't have to do anything until red-DI starts to rise.
but because the red-DI will still be making the TDS drop a bit, your green-DI will produce more than 1000 liters.
So, lets say we grab a figure out of the air and say that for the next 500 liters (after you would have normally thrown it away) the red-DI manages to keep the TDS at an average of 10 TDS, Normally, at 20 TDS your green-DI would be half spent, but now, because it is only being fed with 10 TDS it is only a quarter spent. So the resin in the green-DI lasts longer, many say about 50% longer but of course the science of actual figures is dependent on alot of things (and I do not know them).
What you do is let the red-DI keep feeding the green-DI until you see the green-DI rise. You then know the green-DI is spent and the red-DI is most likely totally spent by now and has the filtering ability of a brick. So you refil the red-DI and change them around. So now the green one, which is producing 1 or 2 TDS and rising, is feeding the brand new resin in the red-DI, so the red-DI produces more than 1000 liters, and so on and so on. You just keep swoping them and refilling the oldest resin.
Hope it made sense to someone who had not quite understood why some use two DI filters.