В документах очень четко указано, что делать.Пример:
const firstAppConfig = {
apiKey: "<API_KEY>",
authDomain: "<PROJECT_ID>.firebaseapp.com",
databaseURL: "https://<DATABASE_NAME>.firebaseio.com",
storageBucket: "<BUCKET>.appspot.com",
};
const firstFirebase = firebase.initializeApp(firstAppConfig, "first")
const secondAppConfig = {
apiKey: "<API_KEY>",
authDomain: "<PROJECT_ID>.firebaseapp.com",
databaseURL: "https://<DATABASE_NAME>.firebaseio.com",
storageBucket: "<BUCKET>.appspot.com",
};
const secondFirebase = firebase.initializeApp(secondAppConfig, "second")
const thirdAppConfig = {
apiKey: "<API_KEY>",
authDomain: "<PROJECT_ID>.firebaseapp.com",
databaseURL: "https://<DATABASE_NAME>.firebaseio.com",
storageBucket: "<BUCKET>.appspot.com",
};
const thirdFirebase = firebase.initializeApp(thirdAppConfig, "third")