Arrange in decreasing order of basicity: (I) piperidine, (II) pyridine, (III) pyrrole.

In piperidine the lone pair on sp3 nitrogen is localised and available, and the ring has +I groups. In pyridine the lone pair is in an sp2 orbital in the plane of the ring and is available, but sp2 nitrogen is less basic than sp3 nitrogen. In pyrrole the nitrogen lone pair is part of the aromatic sextet, so protonation would destroy the aromaticity. The pKa values of the conjugate acids are about 11, 5 and -4, so I > II > III.