When it needs it
I dunno, I mean, what do you mean "clean". I scrape the poo off the droppings board under the roost every morning (elapsed time: 20 seconds) which removes about half the daily poo output from the coop right then and there. Once or twice a year when the birds are molting and I get a lot of feathers piling up, I rake the worst of 'em up along with any pooier spots of bedding and take that out to the compost pile. If I get a wet spot int he litter, like from spilling the waterer, I'll do a scoop or two with the shovel to remove that. Otherwise I pretty much just add litter to what's already there. It is not very time-consuming at all.
With only 3 chickens, as long as you build them a LARGE (for them) coop and LARGE WELL-DRAINED run, sanitation should not be a very big issue. Especially if you use a droppings board which I highly recommend.
I put weeds and garden weedings/trimmings (leaving out anything real toxic) into the run for them to scratch around in. Some people do use hay. Mind they have grit available if you're going to do that, and even with grit available some people have had birds get impacted crops from hay (although many have not; who knows exactly what makes the difference).
Keep the chickens, they're FUN
Good luck and have fun,
Pat