Similar to /amsg..

/msg -e #chan1,#chan2,#chan3 message to send

Would be just like doing using /msg to each #chan, except that it will 'echo' it to each room, the way /amsg does. (The reason for the -e is for compatibility with older scripts, and to my knowledge, irc doesn't allow nicks to begin with "-" so it shouldn't cause confusion with deciding if it's a nick or not.

A similar thread is here:
https://forums.mirc.com/showflat.php?Cat=...amp;amp;fpart=1

Except that is a request to update /amsg