A reading of around 1.007 - 1.010 would probably be around what you're looking for.
EDIT: At 1.020 you are nicely on-track to getting where you want to be.
Have you brewed before this batch? I think I would know if fermentation by the state of the fermenter. Is there a ring of crud above the fluid level, or does it look pretty much the same as it did before you left it for 3 days?
EDIT: Your pictures show the tell-tell signs that fermentation is well underway.