Is there a chance this could effect other apps that these three ports may use? eg - if an app has a range of ports in use like 21000 - 21800 will blocking one within that range cause issues or do connections just re-rout to another number?
This would depend on the OS and application. Typically, if an application tries to use a port that you are blocking, the application is going to error out. If you try again, it might work, if it decides to use a different port number that you are not blocking.
Also when blocking I'm unsure where to enter the address as a Local Service or Remote service (the alert is for traffic heading out so I would imagine its "remote")
For outbound traffic, REMOTE SERVICE would be where the traffic is going to. For inbound traffic, REMOTE SERVICE would be where the traffic is coming from.