Trigger in SQL-How to use query inside the loop?

Question

I am working with TSQL and my task is to write trigger which will be activated every time after something is inserted in table Role and it checks if number of main roles exceeds 3. If that is the case we have to stop inserting.

Here are tables given: Actor(AID, name)(AID is primary key)

Film(FID, name, dateRecorded, numberOfMainRoles)(FID is primary key)

RoleType(RTID, nameOfType)(RTID is primary key)

Role(FID, AID,RTID)(FID and AID are primary keys and RTID is foreign key)

Here is my function:

CREATE TRIGGER addRole
ON Role
After INSERT
AS
BEGIN
   DECLARE CR1 CURSOR
   LOCAL
   FROM
      SELECT * FROM INSERTED
   DECLARE @FID INT
   DECLARE @AID INT
   DECLARE @RTID INT
   DECLARE @NUMOFROLES INT
   SET @NUMOFROLES=0
   OPEN CR1
   FETCH NEXT FROM CR1
   INTO @FID, @AID, @RTID
   WHILE @@FETCH_STATUS = 0
   BEGIN
     ...

   END
   CLOSE CR1
END

For while loop I have an idea, but don't know how to realize it. So I would like to write following query into it:

SELECT rt.RTID FROM 
(SELECT * FROM
Film f, Role r
WHERE f.FID = r.FID) AS table,
RoleType rt
WHERE table.RTID = rt.RTID AND rt.nameOfType = "main"

And then my idea is to check whether the RTID of inserted Role is the same as some of RTID-s that I got in the query above, and then ncrement @NUMOFROLES.

Also, I would probably need to check whether the @NUMOFROLES exceeds the attribute numberOfMainRoles in table Film but not sure how to do that.

Would be very thankful if someone could solve this for me.