oTd:
There should not be a gap between VMCG and VMCA. V1 must ALWAYS be lower than either of these speeds so that the loss of the critical power unit does not lead to loss of control.
For 3 engined take off the performance is calculated on the understanding that the second engine will fail at V1 and the take-off will be continued.
It is a matter of record, certainly for twins, that in most cases air nautical miles per Lb/Kg INCREASE when on one engine. If I can find the numbers from my copy of the B737 QRH I'll post them.