Make sure to use the right subindex for each of the lines:
monitor_specs_0 = "15450.00-16050.00, 55-65, 3.91, 4.70, 6.85, 0.190, 0.191, 1.018, 0, 0, 288, 448"
monitor_specs_1 = "23900.00-24900.00, 55-65, 2.91, 3.00, 4.44, 0.451, 0.164, 1.048, 0, 0, 480, 768"
monitor_specs_2 = "31000.00-32000.00, 55-65, 1.09, 3.83, 2.19, 0.318, 0.064, 1.048, 0, 0, 576, 768"
Also, I'm assuming you're using the same lines in mame.ini, like this:
monitor_specs_0 15450.00-16050.00, 55-65, 3.91, 4.70, 6.85, 0.190, 0.191, 1.018, 0, 0, 288, 448
monitor_specs_1 23900.00-24900.00, 55-65, 2.91, 3.00, 4.44, 0.451, 0.164, 1.048, 0, 0, 480, 768
monitor_specs_2 31000.00-32000.00, 55-65, 1.09, 3.83, 2.19, 0.318, 0.064, 1.048, 0, 0, 576, 768