404 means the server can't find the page. In the case of a dynamically generated page, it can also mean that the page code hasn't received valid parameters, so it returns a 404 instead of giving the user an incomplete page.
You can also issue a 404 manually in htaccess, but ordinarily there would be no reason to do so. If you've removed a page and want to acknowledge the fact, code a 410 for it instead.
Do the "two or three" pages on site A have exact counterparts on site B? If yes, redirect to the best-match pages. If no, well, two or three pages redirecting to a single page hardly falls into "soft 404" territory.
UNLESS! And this is a big unless:
UNLESS "two or three pages" means two or three files on your server, and possibly only two or three URL paths, while in actual page functionality there are hundreds of possible outputs for the human user.