If your flavoring drop size is about 20 drops/ml, then a drop for every 3 would be about 16% flavoring.
If your flavoring drop size is closer to 40, which some are, then a drop for every 3ml would be about 8% flavoring.
Drop size varies by temp, liquid base, flavoring, and dropper style.
But, simply, no nic juice is PG+VG+flavoring, so you're on the right track.