A couple possibilities I can think of. Depends if the products are actually related / have lots of overlapping users, vs. if they’re e.g. totally separate user groups but the forums just happen to be owned by the same parent company.
If they’re related, could make sense to have each product as a category, and use subcategories under those. Could use category permissions so only certain people see certain top level product categories.
Or if you want them as totally distinct forums, probably best to install each forum on a separate subdomain. You wouldn’t have to buy extra domain names but each forum would be separately installed on a subdomain so you’d have e.g. product1.maindomain.com, product2.maindomain.com, and product3.maindomain.com. You could add some kind of shared menu if you want them separate but still interlinked in some way.
For more specific advice would be good to have more detail on what exactly you’re looking to do in terms of how these forums relate to each other, who the users are, etc.