how to debug a trigger in ssms when an app fires it?

I've  created a trigger for a table in an accounting system that should update 3 row given a test that met otherwise return and

do nothing,  but when i test it instead of seeing 3 row which would be updated i see only one. why?

my question is how can i debug this trigger that the application fires when there is an insert of a row(s)?

Is there a way to know the number of rows a trigger inserts or updated?

thanks in adance for you reply

Danny P.