I have your script:
https://www.dropbox.com/s/q0b0l7gb4mc33pf/3%20Rand%20Msgs.rbc
There are blanks to fill in for you:p. It is based completely on randomness, which means, if the game goes bad, it could go into an infinite loop if it randomly chooses a message that has already been said instead of finding a new message. However, his is highly unlikely since there are only 3 options. If you want, I can build in a fail safe to prevent the loop, but it is highly unlikely.
What it does is it checks a var to see if 3 message have been displayed, if yes, it jumps to a message saying that he is done talking. Then, it randmly selects and plays a message and increments the var by one and sets a temp flag. The next time you talk, it will do the same randomly selecting a message. If the message has been used, it will randomly select from the other two, and when it arrives at a message that hasn't been used, it will play it and incremement the var. If it happens to randomly select messages that have already been used over and over, that could result in some lag, but it should find it quickly because there are only three options.
All flags and vars are reset on leaving the map.