Why is a 3° Carbanion unstable than 1° Carbanion? Carbon atom with negative charge will become little electropositive relative to a neutral carbon. So if a Carbanion is attached to three neutral carbons, essentially it should donate it's electrons distributing the negative charge of -1, thus stabilizing it. By this logic, more carbon atoms attached means more stability. But this does not happen. 1° Carbanion is most stable. Why?
Carbanions and Carbocations are perfectly opposite in order of stability because they are opposite in order of charge. The order is preferred of carbocations the same as acidity by Lewis' nucleophile/electrophile definition. Therefore, because a positive charge is stabilized better by a tertiary carbon, it is preferred to host a carbocation because the electron shells of its substituent carbons help stabilize the charge.
In the case of a carbanion, the negative charge is best stabilized when not in contact with neighboring nuclei that would resonate the negative charge away from the nucleophilic site. If you think about it like sterics, the further away the negative charge can be from other substituents, the better in the case of carbanions.