Can't help you with the variable wattage mode; none of my APV's have it. In variable voltage mode, most recommend start low in voltage and increase in voltage in increments until you begin to detect some burnt taste, then knock the voltage down by 0.2 - 0.4 volts.
Another "rule of thumb" is to take your ohm rating and add "2". For example, your 3.0 ohm + 2 = 5 volts.
The Safe
vaping Power Chart can be helpful for new vapors find the resistance range that their
juice delivery
devices can be used.
I adjust my voltage "to taste" depending upon the flavor that I'm vaping at the moment. Fruits often are best in a lower voltage range. Something like a coffee or tobacco flavor may be better at a higher voltage.