Eh, Half-Life 2 was released six years after the first one. A seven year gap between Episode 2 and Half-Life 3 seems about right... If we can only be so lucky to see a sequel in that time frame...
What's interesting though is that Episode 3 was pretty much scrapped, and the total silence since then is just odd. That combined with Source 2 and Valve's recent service based projects implies that HL3 could be very different.