Three kilograms of cocaine. A key
is standard slang for a kilogram of drugs, most often cocaine. By stating "by the three," one is saying that they buy cocaine 3 kilograms at a time with the intention of dealing. The following lines show this more. The statement of a Louis Nap Sack is in reference to a Louis Vuitton bag where he holds all of the cocaine or "work" to be dealt.
The other definition for key by the three is wrong, although the "when I chirp shorty chirp back" is in reference to cell phones.