MYSQL trigger insert into another table for each user from a third table

[Richard Duncan]

I have been trying to figure this out will no good results after researching the internet for a couple of days. It just won't click.

I want to trigger an insert into another table with the information inserted into the main table and have a line for each user of a third table.

Main structure Table A

line_id
room_id
from_name
to_name
timestamp
text_line


Destination structure Table B

line_id
user_name
room_id
from_name
to_name
timestamp
text_line


List of users table Table C

user_id
user_name
user_room
timestamp
status

When there is an insert on table A, an insert is done into table b with the same information from table a for each user_name in table c.

I feel like Einstein. I have found a 1000 ways not to do it.