You didn't mix it enough. Partial boils need a solid 10 minutes of mixing to be a uniform solution. So, your hydrometer sample was inaccurate.
Plug your recipe into some brewing software to get your OG, and also calculate your efficiency. 1.020 doesn't seem that bad for 3 lbs of grain and that much water.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10