parameter-store values show '***' instead of actual value
The "***" is the masking of parameter store output in your log file. The value is available in the environment variable $MONGODB_URI and can be used to pass along to any command or script that needs this value. CodeBuild is merely doing a best-effort masking of log statement which may contain sensitive information.

Also, if you must see the value printed out, you may assign $MONGODB_URI to another variable within your buildspec and print that out.