After Shadow meets Wednesday he is asked to drink three glasses of mead that Wednesday gives him. Afterwards, Wednesday declares that a pact has been sealed between them, binding Shadow to Wednesday’s service. This may be a reference to Odin’s quest for a “magical mead” which would make the drinker a scholar and a poet. The mead is made from the blood of the god Kvasir. Odin procures it over three days, in three draughts. These three draughts could be the same as those given to Shadow. While they do not make him a scholar or a poet, they do represent the pact made by Odin with Suttung, the possessor of the mead at the time. Odin promised to work for Suttung in return for the mead. By accepting the mead and drinking it, Shadow agreed to work for Odin.
This is also where Odin’s symbol, the Valknut, may have originated. It represents Odin’s three horns, or the drinking horns which stored the three draughts of mead.