My grey cells were clearly in a spin when I made my comment about increased duration.
What I was actually trying to convey was that system current would be reduced by a factor of three.
By placing batteries in parallel, the OP was endeavouring to increase battery capacity to 21AH at 12 volts, so total power available would be 252WH and he would be assuming that current load would be evenly distributed across all three batteries.
For example, say motor rating was 200W, then system current would be 200/12 = 16.67 amps. Ignoring any loss of capacity due to discharge rate, the potential duration would be 1.26hours
However, if the motors were in series the battery system would be 36 volts at 7AH. Again total power available would be 252WH. However with the 200W motor, the load current would now be 5.56amps. Duration would remain at 1.26 hours
There are benefits to be gained by having batteries in series (increased voltage)
a) the same discharge current flows through all three batteries so each gets discharged evenly
b) the same charge current flows through all three batteries so each gets an improved chance of being charged evenly
c) by reducing system current by a factor of 3, the I2
R losses would be reduced by a factor of 9 so the system is electrically more efficient.
Apologies for any confusion caused.