If they are identical and your good with hacking the feedback circuits from two of them so that they get their reference signals from the third one its possible.
Most computer power supplies have all of the voltage control feed back referenced to the 5 and 3.3 volt sources. The 12 volt outputs are very loosely regulated and may allow for a considerable amount of power to be pulled off of them when combined without any real ill effects.
Just tie all the same voltage outputs together and see what happens.
The highest output one Will do the most work until it gets pulled down to the point the next one starts putting out power and then they both get pulled down until the third one starts carrying a load also.
Being all three are identical the actual voltage variations between them may be only a be a few Millivolts at no load so they will likely just naturally balance themselves out when loaded down.
But unfortunately the HHO police will be here shortly to arrest this thread from going any further.
(but its still fun to watch though)
Alright. I did test them (without any load) and they were all 12.34 or 12.35. I don't know what happens under load though. At the moment I don't have anything to load them with or I'd check. I'll give it a try though once my 'device' is built.Its worth trying. If you have a good volt meter check the voltage differences between each of them. If its less than around .1 volts from highest to lowest it should work. If not they will likely just shut down and protect themselves. I have paralleled old computer power supplies before without problems.
As far as this being an HHO thread well you did say your electrolyzing water into hydrogen and mentioned needing it to work on 12 volts so it would work with your car.
That could be interpreted as a HHO system here.
I rather like the challenges of non conventional thought and research myself but you will find a few here that are going to get a bit nuts and be rather pushy about their views on it though so be prepared.
Fortunately as long as its kept reasonably scientific it will be allowed here. Most of us do like learning about odd stuff that others are experimenting with.