Well, what do you mean by speaking properly? From an adult point of view, a 3yo is not exactly speaking properly in any language.
Or do you mean, speak it naturally, every day, without an effort? I think this would have to depend on how much they are exposed to it.
I think bilingual parents sometimes tend to get a bit hung up on this and forget that all children find some sounds of their first language hard to pronounce at times, that all children make grammatical mistakes, and that language development goes in jerks, so it is possible to take a sudden leap forward in one language (not always the same one!) and then catch up in the other language. Also, that even monolinguals do not have full command of their languages- there are always areas they don't know much about; so it is perfectly possible to have slightly different areas of expertise in different languages. (e.g. I was a fully fluent academic in Swedish before I emigrated but I had never worked in archaeology until I came to the UK, so all my archaeology is in English. My children can sail a boat in Swedish but not in English- this used to worry me until I realised that their monolingual English friends can't sail a boat in any language).
Both my children live in England and go to school and have only English-speaking friends, but speak Swedish to me at home (most of the time) and spend the summer and Christmas holidays in Sweden. Dh is English.
In the early years their language development was fairly even- but not totally.
Dd (11) is the one who has found it easiest. I cannot remember any time when she was not speaking both languages regularly. But then, I was at home with her, so she got a lot of Swedish from me and English from toddler group etc. I used to spend A lot of time talking to her, singing to her, reading to her, playing with her in Swedish. In fact, her first sentence was a mixture of languages. Her accent is good and she reads both languages for pleasure and is now beginning to write in Swedish.
Ds also spoke a mixture of both at first, but then went through a phase (after a holiday in Sweden) where he refused to speak any English at all! He claimed that if you speak English trolls come and eat you up. Very awkward as he was attending English pre-schools, and his friends couldn't understand what had come over him. But he loosened up a bit when he started school. He still found it a little difficult to switch between cultures, though. I was worried about his accent for a while until I realised that he was having exactly the same phonetical problems as his cousins back in Sweden: some sounds just take longer. He is very happy to speak Swedish now and I am doing my best to make sure that we read more demanding books together etc so that his Swedish doesn't stop at the very basic 'put on your coat' level.
If your husband wants to encourage ds to speak more French I think it would be a good idea to set aside special time when he plays with him in French or reads him stories and tries to encourage him to answer in French.