Anosov flows are rich examples of dynamical systems, they include the geodesic flows on unit tangent bundles of hyperbolic surfaces, and many other examples. This talk is about how dynamics, geometry and topology interact in dimension 3 via some longstanding open questions: Which 3-manifolds support Anosov flows? Which 3-manifolds support many topologically distinct Anosov flows? What invariants can be used to distinguish them? I will describe some of the state of the art, and recent work with Thomas Barthelmé, Steven Frankel, and Sergio Fenley that provides new topological invariants towards this classification problem.