How to detect Double Kill/Tripple Kill?

How can you detect whether a player has killed multiple heroes within say 3 seconds.

Each time he kills someone, he gets another 3 seconds to kill, then another 3 and another.

Basically I wanted to add the UT sounds effects to my map>>

You keep track of when last kills were. You haven't defined the behavior precisely, but a variation on this is probably what you want:
Attach a timer and a kill count to each player.
When a player kills, restart the timer, increment the kill count and give the appropriate message
When the timer expires, set kill count to zero.