Why don't Sony, Ninty and M$ all get together and make one console?

Like this they will be able to compete with PC better and all the good games will be on one console and ofcourse more profits for them

Why would they split the profits between the three of them when one player always gets the largest share of the pie? Nintendo has no incentive to share their billions of profits with two other hardware vendors struggling to break out of the red. Sony had no incentive back when they controlled 75% of the console market.

Why cripple your own company's competitive edge and innovation by tying yourself to two other enormous firms who may not be interested in changing the way things are?

Because MS and Sony seem to hate each other. And Nintendo are sitting on a big pile of money from the wii that they wont want to risk on a joint venture with their two rivals

but if there is one console then each will have 100% marketshare

One: They all have different visions of a console...
Two: I dont think M$ want gaming consoles to 'compete' with PCs.

you are kidding right....
-no competition would be very bad for the console market
-not to mention the console couldnt cater for anywhere near as many people as a range of 3 consoles can
-each company would want to "take the lead" and have their own ideas and it would end badly
-it would get no where near as many sales as 3 seperate consoles combined, which would mean LESS money, less money for devs
-the three companies have different design priorities/principles

i could go on. you clearly did not think this through, OP

Microsoft is only in the business to hinder Sony and Nintendo rejected Sony's buyout offers years ago so it'll never happen. It would potentially be an awesome console though.