My run is 440 sq ft and I currently have 10 birds. But I can definitely see the run floor has built up several inches over the last few years (which is an advantage for me as we've had flooding before). I have a brand new pile of wood chips waiting so I plan on removing maybe half a cubic yard...
So you may still need to do cleanouts I think. 3 chickens isn't much in terms of poop, but it does still build up over time, as both broken down poop and broken down wood all has to go somewhere (it doesn't vanish into the soil, it's more like becomes part of the soil), so this is where run sq...
Any nearby friends/family that garden? We used to take half our chicken poop to my in-laws. In winter I just toss it in the bushes that line our driveway but we have a ridiculously long driveway.
If you 100% have no use for it, it can always go in the trash. Not ideal but can't be helped...