The three legions lost were in Germany, not Gaul. Gaul had been conquered some fifty years earlier and the Gallic nobility was already in contention for senatorial positions.

I don't think there's really a lot to say here. Regardless of the issues of Roman manpower (which increased many, many times over the course of the early Principate, as provincial recruitment became significant and, already by the end of Augustus' lifetime, likely predominated military recruitment, such that by Trajan's time Brunt calculated that only 1% of the legions were being recruited from Italy), this isn't a manpower question. The loss of the three legions in the Teutoburg Forest is not comparable to the losses at Cannae, because the army had undergone a significant change. Whereas in the Republic, particularly in the second century and following, armies would be recruited to meet whatever threat was at hand, and as late as Caesar's time were being disbanded after a few years when that threat was no longer in evidence. Thus the scramble in the late second century to find qualified magistrates and promagistrates who were capable of leading armies, now that the consuls and praetors were insufficient for the needs of the growing empire. Once Augustus won the civil war he disbanded most of the legions, both on his side and on Antony's, and fixed the number of legions, exercising his power of imperium maius, which afforded him total control over all military matters in the empire. Since Augustus was in direct control over all military commanders and all armies in the empire, the recruitment of additional forces was also under his control, and Augustus was extremely wary about the possibility of raising more troops--his own arrival into Italy and rapid rise to prominence after Caesar's death had, after all, been aided by the fact that he had an army at his back. Augustus did raise an emergency levy in the wake of the Teutoburg disaster, conscripting mostly urban freedmen, but he refused to levy additional legions to replace those that had been lost, and our information seems to indicate that his successors also did not levy additional legions, at least not regularly.