many thanks for posting this interesting technical question. As you know, the pyridine ring contains an electronegative (as compared to carbon) nitrogen atom. This makes the pyridine ring comparably electron-deficient. It is also the main reason why pyridine is less prone to electrophilic substitution than many benzene derivatives. On the other hand, nucleophilic substitution (e.g. metalation with organometallic compounds) can occur more readily on the pyridine ring.
The substitution pattern at the pyridine ring can be compared to that of electron-deficient benzene derivatives such as nitrobenzene.
As a result of the electron-deficiency in the pyridine ring, certain electrophilic substitution reactions such as the Friedel–Crafts alkylation or acylation reactions are more difficult to perform or even fail at all. The 3-position is the most electron-rich C-atom in the pyridine ring. This is why electrophilic substitution reactions (if they work) take place in the 3-position.