When looking at Kickstarter game-projects the base goal often says the game will be made for all three PC, Mac and Linux.

As I do not know enough about programming I would like someone who knows to explain: is it nowdays whitout problem to make a game and whitout much work get a PC-port, a Mac-port and a Linux-port?

If so why is it that Steam that tries to promote Linux  for its Steam Machine seems to struggle getting all games in a Linux-port?

Naturally every supported platform, feature and whatever adds effort. But if you rely on a good framework that is available for all three and don't do anything extraordinary, the effort can be minimal. Only testing it and here and there adjust some things. Such frameworks exists, Unity for example.
And for consoles you have to purchase a devkit. This is often very expensive. Probably the Kickstarters add these costs to their calculation and tehrefore make it a stretch goal.

No it isn't without problems at all. There is a significant extra costs in supporting all 3 platforms, especially on the graphics or multithreading side...

Some years ago there used to be a big divide between PC, Mac and Linux but I gather this divide is significantly smaller now. To port to consoles the only problem/cost is the dev kit? As MS Sony and Nintendo all are trying to reduce this cost will it be even easier to port a Unity game in the future. MS claims the retail XboxOne will soon be able to use as a dev kit!

Maybe I should take all my questions to Gamasutra!
Thanks for the info Mnementh and Ail

Ok maybe I wasn't clear enough.

I work in software, in a big software company that makes CAD software ( it's not gaming but the concepts are the same in some way, we display shining polygons with textures and so on..)

Just on the build side we have daily issues getting the same code to build on PC, Linux and Mac ( because PC compilers are a lot more lenient, linux is a lot harsher, and mac just has weird linking rules...)

Aside from that Linux and Windows still don't use the same memory representation ( little endian, big endian) so any optimization you are trying to do on bytes words will have to be OS specific....

Your description of the difficulties in programming for different plattforms seems to coincide with my understanding of the problem. But now Unity, Unreal and Steam gives the info that these problems are in the past. This is what makes me confused!

By the way I used to work with CAD a few years back as a user: primarily with Inventor and AutoCAD.