If I were to recommend three of the four volumes of TAoCP, I would drop Volume 2: Semi-Numerical Algorithms in favor of Volume 4a: Combinatorial Algorithms, Part 1. Developers, junior or otherwise are simply less likely to find themselves in situations where there is a good reason to implement a psuedo-random number generator or a complete numeric system than to wind up dealing with problems of non-polynomial order or boolean functions.

Of course, I don't know why anyone would recommend three but not all four if that were an option, so I guess I really could just say, there's no reason not to get volume 4a too, for a person heading down that road.