Hi Daniele,
You heard right. I lived there 5 years and as far as I know, it is more x3 than x2.
But if you are short, the common practice is to ask for one of your family members, who earn x3, to garanty you. (I don't know the legal term in English, but in French we call this a "cautionnement"). It is very very common in a renting contract in Paris.