If you do that with ferrous raceways, there will be heating effects on the raceways, and higher overall impedance in your conductors. You want one of each phase and neutral where used in each raceway so that their opposing magnetic fields cancel one another and has less heating issues on the ferrous raceway.Yes this is underground, But the Power Company code book says the following:
"Three-phase secondary cables shall be phase grouped in the case ofconcrete ducts and HDPE conduits ( Phase A, B,C and neutral in thesame conduits) and shall be phase isolated in the case of steel conduits(Phase A, B,C and neutral in separate conduits)."
Let me know your thoughts
Brass, stainless, aluminum or non metallic conduits aren't a problem - they are non magnetic.