I'm tryining to find a circuit for an alarm that will latch 'hi' for 10 seconds after it has been tripped for 3 seconds. can anybody help please ?

 

Two timers, one after the other. The first one is your 3 second time delay, the second is the 10 second on time.

http://forum.allaboutcircuits.com/showthread.php?p=91245#post91245