If this is a set of parallel conduits, check to see if all the phase A conductors are in one conduit, the phase B in another, etc.
Does seem warm, but the wires are protected by the breakers thermal trip. If the breaker isn't tripping you should be fine, just losing money do to line losses. Sounds like a continuous duty application without bumped up cables. I would just let it ride, unless there where other negative side effects.
The only two causes I can see would be too-small conductors or the conduit carrying a lot of current.Thanks for the suggestions, no neutral, straight three phase load.