Western Europe - three countries to visit?

I'm planning a trip to western Europe for 12 days and thinking about visiting England, France, and Italy. My friend who's studying in London now (and who's been to all three countries) suggested I should stay only a few days in France as there's much more to see in Italy. I wanted to stay in France longer than Italy but I'm willing to reconsider if that's what most people think and not just her? which is better? I'm a first time visitor... :)