If you Create the 3 SQL Servers in the same PC

Begin

you must use difference port

Principle Server use the Stander port number 5021

Mirroring Server use the Stander port number 5022

Witness Server use the Stander port number 5023

End

Else The 3 SQL in 3 different PC

You can use one port like 5021

Don’t Forget to Disable Firewall or must be open this ports on the Firewall