Goodness Carrie, that's a difficult question to answer! How many days are you travelling for and what will your accommodation be in the Algarve and in Cannes?
I've been to Venice (but not Jesolo), the Algarve, Disneyland Paris and Cannes and so have my children - but not as 3 year olds. My youngest son was 2 years 11 months when we went to Disneyworld Florida (he was only 9 months when we we went to Disneyland Paris) and he doesn't remember it at all now. My oldest son was 2.5 when we last went to the Algarve and he doesn't remember that holiday either.
Your 3 year old will enjoy the beach and also the rides at Disney but what will you and your partner enjoy the most? Disneyland Paris will be the least warm in September, obviously, and personally I wouldn't want to be there more than 2 to 3 days unless you plan to spend a couple of days in Paris aswell. Venice is the most amazing destination but not very exciting for a 3 year old so I guess that choice would depend how many days in Venice versus how many days in Jesolo ...
Where in the Algarve will you be able to stay? If you will be in a beach hotel with pool/kids club etc and maybe have access to a car then you can take your little one to the zoo or to the waterpark ...
Cannes is expensive. The resort is quite lovely and the beach along the coast is child friendly. St Raphael, St Tropez etc are lovely places to visit but will you have a car?
I would probably suggest the Algarve or Disneyland from the point of view of a 3 year old but I think you need to give some more detail before anyone can really offer constructive recommendations.
As a matter of interest, what is your preference?