Italy and Greece in one trip
 

If I have 3 weeks to do this trip, is it doable? does anyone have any knowledge or experience with these 2 countries together?

Its doable but I would rather get to know each of these countries more in depth and get a feel for their culture rather than have just a surface visit to two countries.Plus you'll spend valuable time traveling.

It seems a good fit but seldom turns out that way. I agree with Happy---I would treat these destinations as seperate trips.

Hi; Agree with both replies. But if you decide to do both countries, flights are very reasonable, one way, from Athens to Rome, Milan or Venice. Richard www.kayak.com

I think you could get a good look at a few things if you split the time. You could do a Rome combo with Tuscany or Rome-Amalfi.
I haven't been to Greece yet, but for me, I'd pick less time in Athens and more time on Crete and going around an island or two. I hope you do it!