Carbanion stability is determined by the s-character of the hybrid orbital bearing the negative charge.
The acetylide anion CH≡C− has the carbanion on an sp hybridized carbon (50% s-character), providing optimal stabilization through efficient overlap with the charged orbital.
The vinyl anion CH2=CH− has the carbanion on an sp² hybridized carbon (33% s-character), offering moderate stabilization.
The primary alkyl anion CH3−CH2− has the carbanion on an sp³ hybridized carbon (25% s-character), providing the least stabilization.
Greater s-character enables better penetration of the nucleus and enhanced stabilization of the negative charge.
Therefore, the order is CH≡C−>CH2=CH−>CH3−CH2−.