Greatest Common Divisor.....

Hi guys, just wondering.. how would i work out how to get the greatest common devisor of THREE numbers...not two..but THREE numbers.

for example...

the greatest common devisors for number 29 are "1 and 29".

the greatest common devisors for number 6 are "1, 2 and 6".

the greatest common devisors for number 1981 are "1, 7, 283 and 1981".

so therefor, the greatest common devisor for these 3 numbers is 1.!!

i know how to get the greatest common devisor for 2 numbers, but not 3!

thanks for the help! much appreciated..