You could use a small script.
alias random_message {
var %num = $rand(1,3)
if (%num == 1) return message 1 here
elseif (%num == 2) return message 2 here
elseif (%num == 3) return message 3 here
}
Then you could use /timerrndmmsg numberofplays intervalinsecs msg channeloruserhere $random_message
Example : /timerrndmmsg 0 10 msg $me $random_message
That would message me with a random message every 10 seconds until you disconnect.
If you need more information /help /timer /help $rand /help /alias /help if-then-else /help variables
Those articles fully explain everything you need to improve or edit my script