Craft 3 multi-lingual multi-sites setup with shared assets
You could create 2 site groups. One group for site A and one group for site B. 

Sitegroup A will contain contain 2 sites, the other one only one site.
Your setup will look like this

SiteGroup A
    - Site A - language NL
    - Site A - language Fr
SiteGroup B
    - Site B - language Fr


Let's imagine your Site A - language Fr is your primary site. So it will have the base url /. Your Site A - language NL have the base url /nl.

Your Site B - language Fr will have the url whatever.you.like.fr.
You don't need multiple web folders at all because Craft will route every request to web/index.php and sets the currentSite depending on the url structure.

Now you have multiple ways to setup your asset structure. You can either create multiple Asset Bundles in a plugin and load the assets you need or you can just check in your template

{% if currentSite.id == 1 %}
    load these assets
{% else %}
    load other assets
{% endif %}