Kian,
First of all let me correct you. The handbook doesn't say "3 years". The handbook says "3 years / 36 months". That is an important difference, because PMI is looking for a TOTAL of 36 months of experience. And your example explains it quite well: If you work on projects in 2009 and 2010 then you will have 24 month. No work in 2011 means "no months". But then you work again for all of 2012 means you have another 12 months, so you now have a total of 36 months, which is the minimum you need to have.
So count each calendar month as "1" until you have 36 no matter how many interruptions.
And by the way... the PMI online application system calculates this automatically for you. You will clearly see how much you have or need.