I have set up Gmail API for three different gmail accounts and downloaded credentials.json and stored in visual studio .
Translation, You have created three different projects on Google developer console which will allow you to create three different applications using the Gmail api, and allow users to authenticate your application.
It will also require that you verify three different applications though the Google application verification process. Which can be a costly and lengthy process.
If the Gmail Login Credentials are changed in future , would it affect gmail api credentials ?
That depends on whos login credentials you are talking about.
The user who created the project on Google developer console will still be able to login and administrate the project even if they change their credentials.
Any user who authenticates your application. This is a gray area if you have requested offline access for these users and have gotten a refresh token back and you are using one of the gmail scopes you may need to request a new refresh token as of the user. The last time i checked password changes and gmail scopes require a re-authorization. TBH no matter what your application should be created to request access of the user again if the refresh token fails as they can fail occupationally.
You might find this article i wrote helpful. Google Developer Console Oauth2 credentials
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