You have 2 things at your disposal to achieve this:
- Profiles
- Collections
Go to the OBS menu and under Profiles, select 'New' and give it a name that corresponds to your specific stream. Profiles store all settings data. So use profiles if you want to use different stream sources, video options/bitrates, etc for each stream. If all of your livestreams all use basically the same settings, then you may be able to get away with 1 profile.
Collections will certainly be what you need. In the OBS menu, under Collections, select 'New' and give it a name that corresponds to your specific stream and any scenes/sources you create in that collection will be stores under that collection only. Repeat this for each of your 3 livestreams.
Then you can simply select the Profile and/or Collections from the OBS menu to load your settings/scenes/sources for each specific stream.
Hope that helps!