Assuming you're set up intelligently and are not doing gratuitous work just for fun, three chickens are no more work than three tomato plants. Arguably less
Pat
With only 3 chickens there isn't really much room for having them be different ages. However, they may not all quit laying regularly at the same age, or even all *survive* to that age, let alone necessarily a lot longer. Many do, of course, but many do not.
I guess my suggestion would be...