three friends with different salaries need to find out their average salary without revealing individual salaries to each other. how?

Person A splits his salary into two unequal parts, A1+A2, choosing any split he likes. He tells B what A1 is and C what A2 is. Now B adds his own salary to A1 and tells C the total, B+A1. Then C can add B+A1 to his own salary plus A2 which he received from A, getting the sum of the three salaries, and divide by 3. No randomness needed.

http://mindyourdecisions.com/blog/2009/04/07/game-theory-and-salary-transparency/

The answer can simply be that the first guy adds a random number, and after going around the group the first guy himself subtracts it and gives the average. No need to add two other random numbers as stated above

Ask a 3rd party to take each dudes salaries and return the average salary.

One type of solution is to use a physical token system to record the salaries and then average the total. For example they could agree a system of 10gram of rice represents $1000/year then each friend in turn weighs out the rice equivalent of their salary and tips it in into a communal bowl. After all three are done they just need to weigh the total amount of rice and divide by three. Alternatives would be to use pennies, water volume, etc.