ML model deployment architecture?
No, you don't train against 3 separate databases, because for each dataset, you would need to do the whole work from scratch ( exploratory data analysis , feature engineering etc..). You need a valid training dataset, from which you create your validation dataset and test dataset ( a hold-out set as a final verification step ). Yeah sure, you deploy your model for example as a webservice on a test server to make sure is working as expected. But model verification, including avoiding overfitting or underfitting, having a good accuracy and all that is done prior to any deployment.