Simple
on *:TEXT:Hello:#: {
var %RR $rand(1,3)
if (%RR == 1) { msg $chan Hello $nick $+ ! }
if (%RR == 2) { msg $chan Hey $nick $+ , and welcome! }
if (%RR == 3) { msg $chan Hi $nick $+ ! }

Do you mean to repeat a message every 15 minutes? If so try
on *:text:COMMAND HERE:#:{ if ($nick == YOURNAMEHERE) .timer (Number of times message is stated) (interval in which statement is repeated(seconds) msg $chan MESSAGE HERE }

Try this:
on *:text:!ctt:#:{ if($nick == YourNameHere) .timer 99999999999 900 msg $chan Be sure to click this link to post to your friends on twitter and help grow the channel! It's Much appreciated! http://ctt.ec/adc2V
}