Oxidation is the most important form of chemical breakdown of motor oil and its additives. The chemicals in motor oil are continuously reacting with oxygen inside your engine. The effects of oxidation, due to this reaction, as well as the by-products of combustion, produce very acidic compounds inside an engine.
The various acidic compounds cause corrosion of internal engine components, deposits, changes in oil viscosity, varnish, sludge and other insoluble oxidation products that can cause a performance and durability degradation of your engine over a period of time due to oil breakdown. The products of oxidation are less stable than the original base hydrocarbon molecular structure and, as they continue to be attacked by these acidic compounds, can produce varnish and sludge.
(TBN) Total Base Number represents the reserve alkalinity of the oil to reduce acidic material. Generally speaking, when the TBN goes down to 1/2 of the original TBN number, I like to change out the oil.
The oil I use starts out with a TBN of 12 plus, so when my UOA shows approximately 6 TBN, I change it.
As to the 3 Q sump capacity, I think Oil Analyzers uses "Gallons" as capacity, so the 3 would indicate 3 gallons.