I'm planning to start and end my trip from Istanbul.

in between 12 days I would like to visit Italy - London and Germany

Expected travel will start beginning of May.

I am a budget traveler

If some one can help with some kind of itinerary would be helpful for me.

I will be travelling alone and have few friends on in Venice, Berlin and London.

Nazmul

Budget, then you need to look at the budget airlines skyscanner.com might help and Hostels for accomodation.

12 days to see a big area, so you need to focus down on cities, are we talking Berlin and Venice

To keep costs down and make the most of a small amount of time you may prefer to use night trains. set61.com may be some help in this area

You also have France in your title. So please clarify, do you just mean London, Venice and Berlin . . . or some other places too?

"Budget" and lots of moving from place to place are conflicting concepts.

12 days: London, Berlin or Munich, Venice. You do not have time for more.

Mike is right: London, Berlin and Venice; there is nothing to add!