There is no reason why you can't do all three languages at once! LR is a great way to enforce language learning - it would be easiest to start with English as you just need to buy the ready-made curriculum.
Maybe your husband could do the French flashcards, you do Portugese and at the same time do English?
I am using LR for English, am preparing Russian slides to start in a couple of weeks (so we don't start just before a holiday) and we've got the trial of Chinese which I would love to upgrade, if I can find the funds!
Personally, I wouldn't do the exact same set of words in both languages simultaneously, because it gets boring (for me at least
) seeing the same pictures over and over, but delay 10-20 days to allow the sets to be different. That will also give you plenty of time to create the slides for each subsequent language.
It might be an idea to prepare some samples in all three languages and let your baby choose! I know my son prefers other languages over English any day - maybe your son will have a preferred language to listen to, in which case - start there!
It's nice to see another UK based mummy - there are so few of us around