PGC, Deeper would be better as 3 ft depth will get you only about 500 gpm turnover with the right diffuser.How far away is your power as there are some inexpensive pumps that can pump air for "miles" no electric nearby needed.As far as a windmill in 3 ft of water be very selective on a diffuser as the standard 7 inch Koender type airstone may not give much open water and perhaps 100-200 gpm turnover with 1 cfm.What solar system are you looking at??