If it is 3' high you can probably reach 4' with a rake without too much pain or trouble. Either 4x8 or 4x10 would give enough room for three chickens, especially considering you will also have a coop. The trick would be coming up with a way to access it. Maybe a frame that you can bolt on (butterfly nuts have come in handy for me) or a hinged door placed where you can still access the corners. If it is just for cleaning you will not be using it that much.
Trial and error can help you find out what works and what doesn't if you are willing to go that route. You might be OK with it, you might not. But the worst case would be more of an inconvenience for you, not something fatal to the chickens.
Trial and error can help you find out what works and what doesn't if you are willing to go that route. You might be OK with it, you might not. But the worst case would be more of an inconvenience for you, not something fatal to the chickens.