Campaigns cannot be created in a manager account. It can only be created in a client account. This is true for both AdWords UI and API. You can check this example (in Java) on how to create campaigns.
Refresh token is not campaign based, it is user based or email address based. You can check these users in your AdWords UI -> Account settings. For example, if you have 2 users (e.g. a...@example.com, b...@example.com) in your client account, then you can generate OAuth2 credentials (with client ID, client secret, refresh token) to these two users. The user a@example will have a unique refresh token and b...@example.com will have a different unique refresh token. The idea here is that OAuth2 credential is associated to an email address of a user which is under your client/manager account, and not on a campaign or the client account itself.
You can check this guide on how to authenticate in the AdWords API including the generation of refresh token (see this section).
Could you confirm on why do you want to generate separate refresh tokens to your campaigns or to your client accounts? If you want to access multiple client accounts under an MCC, then you can select a user from that MCC that has administrative/standard access, and generate the OAuth credential for that user. This way, your credential can access the MCC and all sub MCCs and client accounts under it.
Yes, it is mandatory to have client ID, client secret and refresh token to generate the access token. As I have said, the access token is automatically generated in the background using the client library. Client ID and client secret are needed to generate the refresh token, and the refresh token is needed to generate the access token, so all these properties are required.
It is not the refresh token alone that is needed to access an AdWords account. Every refresh token is associated to a client ID and secret, so you cannot randomly match your client ID and secret with the refresh token (e.g. from the other email address).
To answer your question, if the refresh token is the correct refresh token associated to your client ID and secret, then you can access any AdWords account as long as the email address (associated to the client ID) is a user with valid permission in those AdWords accounts. If you will be using a refresh token that is not associated to your client ID, then the API will generate an error. If you'll target an AdWords account and the user is not existing on that account, it will also generate an error.