Hi Byron
Just to summarize your question- you want to run three copies of MongoDB locally, and 1 in the Cloud, correct?
I came across posts with similar questions such as this and this. Do this help answer your question?
Here are a couple of resources that explain how serverless pricing works: resource_1 | resource_2
Can you tell me more about why you need to “add a replica set”? What are you trying to achieve?