Unless everyone's Killzone 2's disc detonates the moment that Killzone 3 releases, I would imagine that there is more than enough time to get things done. The community may fade more quickly with the release of a new installment, and, 6 months to a year down the line, the statistical chances of being in the top 1% will probably dwindle meaning you'll have to work harder, but I think that is common sense. However there is no indication whatsoever of the servers being closed down and I wouldn't bank on the possibility of signing on the first complete week of March only to find 3 people on Killzone 2's multiplayer.
Most popular, (and especially exclusive) games keep the servers up for a long time. I wouldn't stress yourself out this much about it; one day at a time.