A projective 3-manifold is a smooth manifold that admits an atlas with values in the real projective 3-space such that all transition maps are restrictions of projective transformations. A M"obius 3-manifold is defined similarly, with the projective space replaced with the standard 3-sphere and projective transformations replaced with M"obius transformations. (Recall that a M"obius transformation of the sphere $S^n$ is a self-diffeomorphism of the sphere that preserves the angles of the standard metric; such transformations form a Lie group isomorphic to $SO_{n+1,1}(\mathbf{R}))/\pm I$).

Both projective and M"obius manifolds are particular cases of manifolds admitting an $(M,G)$-structure in the sense of W. Thurston.

Every closed (=compact, orientable and without boundary) 2-surface admits both a M"obius structure and a projective one. I vaguely remember having been to a talk some time ago where the speaker said that (conjecturally?) the situation in dimension 3 is similar. But I don't remember the details at all. So I would like to ask if anyone knows whether either of the statements (each closed 3-manifold admits a M"obius, resp. projective structure) is a theorem, a conjecture or becomes one or the other after eliminating some counter-examples.

A related question: if memory serves, in the same talk it was mentioned that the $PGL_{n+1}(\mathbf{R})$ and the M"obius group are (conjecturally?) the maximal groups that can act faithfully on an $n$-manifold. I was wondering if anyone knows a reference for this.