Experiments indicate the importance of three-dimensional action during
transition, while high-Reynolds-number-flow theory indicates a multi-
structured type of analysis. In line with this, the three-dimensional
non-linear unsteady triple-deck problem is addressed here, for slower
transition. High-amplitude/high-frequency properties show enhanced dis
turbance growth occurring downstream for single nonlinear oblique wave
s inclined at angles greater than tan-1 square-root 2 (almost-equal-to
54.7-degrees) to the free stream, in certain interesting special case
s. The three-dimensional response there is very 'spiky' and possibly r
andom, with sideband instabilities present. A second nonlinear stage,
and then an Euler stage, are entered further downstream, although fast
er transition can go straight into these more nonlinear stages. More g
eneral cases are also considered. Sideband effects, sublayer bursting
and secondary instabilities are discussed, along with the relation to
experimental observations.