How about an "Aloha" theme, since it means both hello and goodbye? 
The person leaving has he/she been their a long time. I would make two cakes the same size (small cake is fine- same flavor or different)- one for the person - reason for leaving - retirement, new job, etc theme. The other cake is to welcome the two new employees. My reason they are two different celebrations, you can keep the color scheme similar but you are giving each a good goodby and welcome. Just a thought
I like the aloha idea too...but just to add another idea...I saw a cake here on CC that was for someone leaving. here it is:
http://cakecentral.com/gallery/2051173
You could do two round cakes...one with the happy face and one the sad.
Could you do something like this? Clearly modifying the words... if you could still make them work so that when it is read from one side it says Goodbye/Goodluck or whatever and the other side it reads for the new comers?
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