With properly wired 2 hots, neutral and an EGC, there is no magnetic field outside the conduit unless there is a fault. All current going out on either hot wire returns on the neutral or other hot. With the same current going opposite directions, any induced magnetic fields are cancelled. So there is no concern about induction. The 208V three-phase makes no difference.
There might be a spacing concern on the conduits, but not likely. Derating for grouping is not explicitly called out in the NEC. The Okonite cable handbook and other sources have derating tables for grouped conduits where spacing between conduits is < conduit diameter. Example: a horizontal rack of six 2" conduits with 1" space should be derated to 0.86 for one row, 0.79 for two rows, 0.74 for three rows. Three conduits vertically stacked are derated 0.85. (The table does not apply for spacing less than 1/4 diameter).
I would not worry about derating for your case. The table is based on derating cables from the manufacturer's stated free air ratings, which are usually higher than the NEC allowable amps. Table values also assume 100% load factor with all circuits operating at full ampacity 100% of the time. Other factors are used to account for load diversity and variable loading over time.
The NEC tables probably account for these factors already. If they didn't, there would be a lot of melted circuits out there.
