I actually think of this all the time. I think I'd have more time to grab just three things, but that's because I have important things in the same place just in case of an emergency. I have my "special box" which is a drawer in my desk that has all of the small but important things in my life. I have a note from my father, old plain tickets, what's left of my old Pokemon cards, and much more. I find some of the small things more important than a Wii or my laptop, so I'd take them.
If fire was spreading in my house and I could only bring three things, I'd go to that drawer first. I'd grab all of those belongings (assuming the drawer counts as one thing) before moving on. The second thing that I'd grab would be my photo album. In sixth grade, I took all of the pictures that I have of my life and put them in order. I would want my children to be able to see my roots. The third thing I'd get get would be my diaries. Again, I would want my children to be able to see my roots and have guidance from the mistakes that I made. These three things are under the assumption that my family and pets could get out safely.
If I could grab a couple other things, I'd get some of my clothes, since they're incredibly important to me XD;. I'd also get my DS with my Diamond, Platinum, Sapphire, and Fire Red versions, 'cause I put a lot of effort into them and would need an escape after everything of mine was destroyed. I'd grab my two stuffed animals, Baby Arcanine (shut up, I was six when I named it) and Alberto the Frog for comfort. If I could get my laptop, I would, but it's not of huge importance. Before dashing out the front door, I'd grab the little basket in the livingroom with all of my family photos in it. Wow, it'd be impossible to just choose three things. :/