An equilateral-triangular shape of three α clusters surrounded by excess neutrons is suggested for 14C, based on the molecular-orbit model. It is found that the attractive interaction between an excess neutron and an α particle stabilizes the Kπ = 0+ and 3－ rotational bands, which demonstrates an equilateral-triangular symmetry. This Kπ = 3－ band at 3 MeV below the 10Be + α threshold energy corresponds to the experimentally observed band built on top of the second 3－ state. A positive-parity rotational band (0+, 2+, 4+) arises similarly. These two bands suggest a molecular 3-α structure stabilized by the excess neutrons and can be viewed as a realization of the α crystallization in the dilute nuclear medium.