I had a great meal here when I was in Cannes. It was one of the few places that had a clearly-marked vegetarian meal on the menu. Les 3 Portes was also one of the few places that served a vegetarian meal other than Italian-inspired pasta and pizza. The food is quite pricey, but the prices are pretty much on par with the other fine restaurants in Cannes.
The Vegetarian Tajine was superb. The waiter brought me a large pot of it (more than I could finish) along with a separate bowl of semolina to eat it with.
The sorbets were also quite good. I would recommend the Willamine and Pear over the Vodka and Lemon sorbet, but that's just a matter of preference.