In this paper, we present a global adaptive output feedback control scheme for a class of 3rd-order uncertain nonlinear systems to which adaptive observer backstepping method may not be applicable directly. In contrast to the existing output feedback form, the allowed extended output feedback structure includes quadratic and multiplicative dependency of unmeasured states. Our novel design technique employs a change of coordinates and adaptive backstepping. With these proposed tools, we can remove linear and quadratic dependence on the unmeasured states in the state equation. Also, the multiplication of the two unmeasured states can be eliminated. From the transformed systems, a state observer can be constructed in a very easy way. The overall scheme achieves globally exponential convergence of the tracking error to zero while maintaining global boundedness of all the signals and states.