AFAIK the standard configuration is a 120-0-120 supply with the centre-tap grounded, serving all single-phase consumers. If a small 3-phase supply is required, a 208V high-leg (the B-phase) can be brought in to make 3-phase 240V. Being an edge-grounded delta system, 120V supplies can't be derived from centre-taps in the other two pairs of phases as these would not be grounded and therefore not suitable as single-phase neutrals. The system is therefore intended to run with unbalanced loading on the LV side, and indeed sometimes supplied in open-delta with a main single-phase transformer serving A & C and another single-phase transformer for the less loaded B-phase.
A separate 3-phase supply in one of a number of different voltages and configurations is specified for commercial users where the high-leg system would be insufficient or impractical. The variety of different voltages and non-interchangeable plug types is baffling to the European onlooker accustomed to 400/230V supplies. See:
Tables of NEMA plug and socket types for various US electrical supply configurations
In any event, most US single-phase electronics that are not universal voltage, are 120V.