Yes, you should be thinking 3-5 years ahead. And if you're thinking that far ahead, you'll amortize the free 3 years with the 2 years after in which you pay and come out with a true cost. You might even work in the odds that you'll be in business and/or have sold out in that time along with some estimate of switching costs to other technology (which might actually be low if you mainly use Azure).
I don't know if it still comes out as worth it, but that's what you have to figure out. A kneejerk "I'll be paying for this in 3 years so I shouldn't take advantage of it being free now" isn't exactly the most rational response either.