You have: 100ml, 0mg nic.
You want: 120ml, 360mg nic (120ml * 3mg/per)*
You add: 3.6ml nic solution (360mg / 100mg/ml)
You now have: 103.6ml, 360mg nic.
You want: 120ml, 360mg nic.
You add: 16.4ml PG/VG
You now have: 120ml, 360mg nic, for 3mg/ml total concentration.
*(I'm assuming you meant...