Hi Mike,
When folks compare camera specifications they often give weight to one parameter being better than another, without stopping to consider whether a given parameter with either camera is "more than adequate" (and thus the decision should be based on something else).
Pixel counts are a prime example; if I display one of my images here at a generous 800 x 1200 pixel resolution then I'm using less than 1MP - if I print an image at a typical competition size of 12 x 8" @ a more than necessary 300DPI then I'm using 8.6MP - so assuming that one has framed a shot reasonable well (ie isn't cropping more than - say - 1/3 of the image) then about the only difference one will notice in shots from cameras greater than 12MP is the higher percentage of pixels that are discarded. Personally, I shoot with a 21MP camera - and typically discard over 95% of the information captured - and STILL have more than enough to present to the world. Along similar lines, folks often use the example of printing large prints as justification for high MP counts, but even this doesn't really stack up; because prints are 2 dimensional, one needs 4 times the number of pixels if one want to double print resolution (ie if you have a typical 18MP camera and you want to double the resolution you'll need a 72MP camera) - but even that is a moot point because as the print size increases, so does the viewing distance - and thus the less detail our eyes can resolve at that distance. So again - once one goes beyond the 8 to 12MP range it's pretty much "academic" as opposed to "real world" photography.
Low-light is another discussion; I do a lot of low light photography (up to 40 minute exposures) in areas where it's so dark I need to be careful not to trip over my tripod leg whilst wandering around aimlessly waiting for the exposure to finish. In reality - unless you need to hand-hold the camera in extreme low-light - then it'll be secured to a good tripod and any of the 3 you mention will do the job just fine.
Some suggest handling different bodies and see what feels best - but in my opinion even that doesn't carry a lot of weight; I'm sure if I were to take a 430km/hr Bugatti Veyron for a drive then it would feel pretty weird compared to my Ford Telstar, but that doesn't mean my Telstar is the better car -- it simply means that I need a couple of days to get used to something different. Cameras - or rental cars - or many other things in life are no different.
So what do I suggest? In reality - if you don't already have an investment in lenses for one brand over another then it honestly won't make ANY difference. People sweat about making the "wrong choice", but in reality there is a HUGE overlap between models such as the 3 you mention. If you have the money then the D800 and 5D3 will both serve you well - but keep in mind that the camera body is only a minor part of what you'll need in the future. You'll also need lenses - flashes - tripods - filters - a remote release - storage cards - processing software etc. If all of that is starting to stretch the budget then a 5D2 may save you enough money to buy something else for your camera system.
Hope this helps