@Syakster There could be a couple of things in play here. Kayfun tanks tends to use higher resistance coils. In your Subtank, are you using the 1.2 ohm coil or the 0.5 ohm? Subohm coils produce more vapor therefore the effect of the nicotine will be higher.
As for the TC mod, in my perception, TC mods tend to hit a lot smoother since the TC mod will limit the power to maintain a set temperature. I have found that just because you're using TC, that does not necessarily reduce the throat hit from the nicotine. You mentioned that 3 mg juice isn't available where you are. How about 0 mg juice? If you have 0 mg available, you can simply mix it half and half with the 6 mg and that will get you 3 mg.