Think I can get 3 years out of a GTX 560 ti 448?

Yes, If you were trying to make it to Maxwell and skip Kepler like i'm doing (SLI GTX 560 ti's) I would also agree you could make it another 2-3 years. But just make sure your power can handle it. That card will Handle 1080p games excellent. You may not get excellent fps on all games then your gonna need a better card. but it holds it's weight on most all new games.

Will the 560 Ti 448 play most games released in the next 3 years = Yes.

Will the 560 Ti 448 play games released in the next 3 years on high settings @ 1080 = No chance.

That doesn't mean you couldn't get 3 years out of it, but you will have to lower the graphics on most games.

Let's do different...
The flagship GTX 285 is a 3-year old card today (launched Jan/2009)
Would you be happy using one of it today? (It's performance it's rated like a GTX 460)

If yes, think that we're talking about a 3 yr old FLAGSHIP, what about a mid-high card like the GTX 560Ti-448?

I would say, if you want to keep details at high for the next 3 years, you need to buy at least the GTX 580..

Well I bought an XFX HD5970 Black just over 2 years ago and it's still fantastic. OK it was the fastest card in the day but with the way it performs now I have no intention of upgrading until after the 7000 ATi or Kepler. By which time it will be at least 3-4 years old. I'm thinking if you want high settings for the next 3 years you might be best with a 580 but good luck anyway!