First of, I guess welcomes are in order, so welcome to SH3...
Now, I would just like to make things clear...
when you say campaign do you mean career or single player missions???
oh and yes... friendly ships will engage enemy ships... but in career it is not so often you would find a surface naval engagement as you are a lone U-Boat commander who would be assigned to patrol different sectors, but you can still find other ships and follow then as their escort.
german U-Boat commanders were given a lot of freedom in how they see fit to go about these patrols, in other words you don't always have to go to the assigned patrol area and you basicaly can decide how you want to go about your patrols.
Where as in single player missions you are mostly directed to the battle and there is more action going on...
So, if you are a player with patience and a player who wants to feel as if though he was in WWII than career is for you...
But, if you don't want to wait around so long and do what a Captain of a ship would normally do like plot couse manage crew and U-Boat specification, than single player mission is for you...
I don't know much about SH3 commander as I don't have it yet...
sorry...
I hope this was what you were looking for, and also I could be wrong in some way, but I'm sure someone will correct me...
Happy Hunting...