I imagine it's safe to assume it won't appear on the 360 & PS3 despite their early intentions. I'm sure the time between the first trailer if it's released on time and final release will be a lengthy one. And it's hard to imagine Half-Life 3 releasing on last gen hardware that's several years out of date by that point.
But if this is a heavily massaged and improved Source engine, maybe it will. That has shown to be very capable of being scaleable over the years and able to run well on lesser hardware. So maybe I shouldn't be so quick to dismiss it.
If large numbers of 360 and PS3 gamers stick to their hardware for several years and routinely are buying new releases, maybe it's possible. But I imagine that bit about last gen hardware will quietly be forgotten by Valve by the time 2015, 2016, or 2017 rolls along and HL3 hopefully releases.
I can't imagine a Fall 2014 release actually being possible unless there's a lot more work done than they're leading us to believe. And past 2014 other than for franchises that are essentially yearly updates at this point like Call of Duty, I think 360/PS3 support for major 3rd party releases will start to get rather thin.