You're describing a more advanced way of reverberating. Beat suggested to set 3 various settings for 3 different depths - to achieve that with one external unit you simply have to render three (dry) tracks each containing the instruments you want to have to reverb in that part, e.g. the brass and timps and percussion should be rendered together to one file, then set with reverb 3 far back in the stage.
What you described is the IMO even more realistic use of it after that step with adding only as little reverb as necessary to create the right depth between the 3 sections you could render all the new 3 reverberated files into one and play that through your (one) external reverb unit. That way you can control the ambience and give it a certain feel, like the Concertgebouw for example. You could also try to get your hands on one and sample that to use it with a convolver like SIR or in Waves. That is also a way to achieve it without the hardware and all at once if your PC is capable of it.
Hope this helps,
PolarBear
What you described is the IMO even more realistic use of it after that step with adding only as little reverb as necessary to create the right depth between the 3 sections you could render all the new 3 reverberated files into one and play that through your (one) external reverb unit. That way you can control the ambience and give it a certain feel, like the Concertgebouw for example. You could also try to get your hands on one and sample that to use it with a convolver like SIR or in Waves. That is also a way to achieve it without the hardware and all at once if your PC is capable of it.
Hope this helps,
PolarBear