If you are going to use your projector for a lot of hours, 20 hour per week or 1000 hours a year, then a 3 year warranty will probably pay off. 20 hours per week is 10 movies - that's a lot of movies to watch. If it's your main HD machine, well then thats a different story because you will watch more than just movies combined with TV.
If you are just using it for a few movies a week, then the number of hours probably isn't a good deal.
Also, in 3 years, do you really want to still have a HD65 as your projector? If you plan to move up, then a new bulb isn't really very useful or economical.
At $100 you are already paying for 1/2 the price of a new bulb, so maybe its better to just bet that you won't need a new one and you will save $100. If the bulb does go then you have a good excuse to look at another machine in a few years.