Going off of what I think are hints with the icons, the first is revealed to be SteamOS and is a circle. The second is likely this "SteamBox", a branded computer hardware bundle that comes with the operating system and an attempt at making a standardized gaming computer, similar to consoles, that developers will use. If successful, it'd annul one of the advantages of console gaming, which is standardized hardware (if you own the console, it will play any game released for it).
The third circle, two circles with a plus sign in-between, is an attempt to throw people off of the first circle and the second circle in a box. Instead of the circle relating to the SteamOS, I believe that it's going to be a specialized gamepad controller (the circles being analog sticks, the plus sign representing either the dpad or the four buttons). A lot of people just use XBox controllers, but seeing that Steam is making their own OS and their own potential console for computer gaming (my guess), all they're missing for the living room is the other advantage of console gaming: some games just work better with controllers. Every console release also comes with a controller release, and I bet the logic is that they should release a controller as well, as the XBox controller was never designed with PC gaming in mind (not that it necessarily needed to, but whatever).
If I'm right, I'm hella excited. I'm a fan of console games but have never been into the console scene, and I support the PC side of the whole console vs. PC thing. This will remove a bunch of wind from the sails of the console argument (which I acknowledge and agree with), AND, it opens up the potential to see what would otherwise be console exclusives on this new standardized PC platform. I'm not sure where I stand with relation to Valve, but considering the outrageous success of Steam, I actually think they could pull this off. I'm rooting for them.