Just-for-Fun relationship or Happily-Ever-After chick?
If Just-for-Fun then be the crazy uncle. Visit and play a little but never get too close or too involved with the kid.
If Happily-Ever-After then be ready to be anything from a grandparent (if the biological father is still active) all the way to a full-blown parent. If the biological dad is decent and still in the picture, you get to be the grandparent! Don't ever come-down on them. That's mom and dad's job. And when you do things with them, just have fun. Now if dad has been missing for three years, you're gonna be a real father. And if that's the case, make sure you're ready for the responsibility because you're marrying both that woman and her kid.
Having said all that, being a real father is amazing. I wouldn't trade it for the world and I wouldn't let a kid stop me from the right woman at any cost.