Consider I 1-methylcyclopentyl cation, II cyclopentyl cation and III bicyclo[2.2.1]hept-1-yl cation (cation at the bridgehead carbon). The correct order of stability is:

I is tertiary (more alpha hydrogens and +I groups) and planar, so it is more stable than the secondary cation II. In III the bridgehead carbon is held in the rigid bicyclic frame and cannot become planar, so it is extremely unstable even though it is attached to three carbons. Order: I > II > III.