  We provide families of affine threefolds which are contractible in the
unstable $\mathbb A^1$-homotopy category of Morel-Voevodsky and pairwise
non-isomorphic, thus answering a question of A. Asok and B. Doran. As a
particular case, we show that the Koras-Russell threefolds of the first kind
are contractible, extending results of M. Hoyois, A. Krishna and P. A. Ostvaer.
