Corn yields perhaps 2.8 gal/bu and averages ~150 bu/acre, so you're going to see around 420 gal/acre. Anyone claiming 3000 gallons from corn is smoking something.

That is for oone crop. Maybe some areas can have 3 crops a year, assuming 350 bushsls/acre and 3gal/bu, that would work. But it seems to be a stretch.

BTW, How much biomass is in the cobs and stalks, compared to corn kernels?
To me it seems that there is more potential in processing the "waste".

That is for oone crop. Maybe some areas can have 3 crops a year

What strain of maize can come to maturity in 120 days at that yield?  With enormous amounts of water, phosphate and nitrate I suppose you can do anything, but algae seem cheaper.

How much biomass is in the cobs and stalks, compared to corn kernels?

Around half (maybe more if cobs are included, I haven't checked lately).  A bushel of corn is 56 pounds, so 150 bu/ac is 4.2 tons.  The Corn Stover Collection Project found roughly 2 tons/acre of harvestable stover, after allowing for erosion control.