You can check that those pages aren't set as multiple homepages.

You can check the homepage setting.

• Settings -> Home
• https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

Firefox supports multiple homepages separated by '|' (pipe) symbols.

Two other reasons extra tabs might open:

(1) If they are "pinned"

Pinned tabs always re-open in the first window at startup. More info on this feature: Pinned Tabs - keep favorite websites open and just a click away

(2) If Firefox is restoring your previous session

Firefox may restore your previous session based on your setting on the Settings page, or if it perceived that it crashed instead of performing an orderly shutdown.

Since they are always the same 3 tabs, that seems kind of unlikely, unless it is an old session that keeps getting restored for some reason.