How do remaining two phases act as return path in three phase system
With the three phases being out-of-phase with each other by 120°, there can never be all three phases at the same voltage.  So if, at any given instant, L1 and L2 are at the same voltage, then L2 and L3 will be different, and L3 and L1 will be different.

Three-phase loads normally consist of three elements - heaters, motor windings or whatever.  So there will always be a current flowing through at least two of them.

A current will always flow if you connect a load between two points that are at a different voltage.  It doesn't matter whether you choose to call them "phases" or "neutral", the electrons don't care.