For a manifold M with an integral closed 3-form, we construct a principal PU(H)-bundle and a Lie groupoid over its total space, together with a curving in the sense of Murray. If the form is non degenerate, we furthermore give a natural Lie 2-algebra quasi-isomorphism from the Baez-Rogers observables of M to the weak symmetries of the above geometric structure, generalising the prequantisation map of Kostant and Souriau.