Three is not very strong. 6 is stronger, the higher the number, the more nicotine is in the eliquid. I'd go with either a 12 or 18 and mix with what you already have to get stronger than six mg. Just to be on the safe side, get the 12 or 18 and use that. If it's too strong, dilute it a little in a different bottle a little at a time with the eliquid that you already have (3 or 6) so you don't screw up the original amount of eliquid.