How to authenticate many users with the same public key.

Hi everybody.

I would like to ask you about one thing. My company owns almost 100
servers. I would like to implement key authentication for all
administrators (3 people). The problem is when I have generated public
key, it works only for my PC. I would like to have a one key for
everybody. Is this possible ? I know it should be because I did it
once, but I don't remember now how and what have I missed this
time :/.

Can anyone show me good direction ?

Thanks for your help in advance.