The primary difference relates to the instantaneous power of the two systems. The instantaneous power from a single phase system is more or less sinusoidal, with periods of peak power, periods of low power, and even periods of negative power (depending on the powerfactor).
The instantaneous power from 3-phase is always constant.
In the graph below, you might remember seeing part of this when I described the instantaneous single phase power in a previous discussion. The voltage and current curves represent the single phase current and voltage with a 0.6 powerfactor. The dashed purple line is the single phase instantaneous power.
I used the same graph, but added the B-phase and C-phase power based on the original voltage and current, and then I added all three of these power values together. The total instantaneous power is the sum of the three phase powers, and is shown by the straight dashed line at the top of the graph.