Let $X$ be a smooth contractible affine algebraic threefold with a nontrivial algebraic ${ \bf C}_+$-action on it. We show that $X$ is rational and the algebraic quotient $X//{ \bf C}_+$ is a smooth contractible surface $S$ which is isomorphic to ${ \bf C}^2$ in the case when $X$ admits a dominant morphism from a threefold of form $C \times { \bf C}^2$. Furthermore, if the action is free then $X$ is isomorphic to $S \times { \bf C}$ and the action is induced by translation on the second factor. In particular, we have the following criterion: if a smooth contractible affine algebraic threefold $X$ with a free algebraic ${ \bf C}_+$-action admits a dominant morphism from $C\times { \bf C}^2$ then $X$ is isomorphic to ${ \bf C}^3$.
Categories: math.AG