Those three examples you gave will result in dramatically different vape experiences. This is a little long, but bare with me.
Check out this chart:
Let's start by looking at a coil with a resistance of 1.4 ohms. (I'm using 1.4Ω instead of 1.2Ω because 1.4Ω is as low as the chart goes and it makes it easier to explain.) If you set your variable voltage device to 3.4 volts, you end up with a power level of 8.26 watts, and you are on the high side of the green "safe power" area.
Next, look at a coil with a resistance of 2.0Ω. If you keep your VV device set to 3.4V, you end up with a power level of 5.78W. In this case, you are now on the lower end of that green "safe power" area. You'll end up with less vapor production, less of a throat hit, and less flavor with the same eliquid.
Finally, look at a coil at 3.0Ω. Again, if you were to keep your device set to 3.4V, you would only end up with a power level of 3.85W. Now you are almost on the border of the yellow "underpowered" area of the chart, with a much weaker vape: even less vapor, less TH, and less flavor.
Now for me, I use my mod in variable wattage mode and I normally keep it set to 8W no matter what the resistance of my attached coil is (give or take 0.5W or so, depending on the eliquid I'm vaping). Doing that results in a more consistent vape experience no matter what atomizer is attached to my mod. To illustrate, let's now look at the voltage that my mod sets for me with each of the three coils we just looked at when I set the power to 8.0W:
1.4Ω @ 8.0W = ~3.35V (toward the right/high side of the "safe zone" in the chart)
2.0Ω @ 8.0W = 4.00V (again, toward the right side of the green zone)
3.0Ω @ 8.0W = ~4.90V (the same, the high end of the green area)
In order to get a similar vape experience with coils of different resistance you need to raise your voltage as your resistance increases and lower your voltage as your resistance decreases. Using variable wattage, you can set your mod to output a specific power (in watts) and whether you are using a 1.4Ω coil or a 3.0Ω coil doesn't matter; the mod automatically raises or lowers the voltage in order to hit that set wattage (8.0W in my case). If you are using a 1.4Ω atomizer and decide to switch to a 3.0Ω atomizer and keep your wattage setting the same, the mod will automatically increase the voltage from 3.35V to 4.9V. That's a pretty big difference.
If it makes it easier, think of the power output in watts as the amount of heat generated by a coil. As the resistance of your coil increases you will need to send more voltage to that coil in order to achieve the same amount of heat, and vice versa.
Now, as to your question about battery life, the fewer amps drawn from the battery the longer the battery will last before it needs to be recharged. You can calculate the number of amps (A) by dividing the number of volts by the resistance of the coil. So,
1.4Ω @ 8.0W = ~3.35V = ~2.39A
2.0Ω @ 8.0W = 4.00V = 2.0A
3.0Ω @ 8.0W = ~4.90V = ~1.63A
Therefore, at 8W of power you will be able to fire your 3.0Ω coil the longest before the battery drains.
Hopefully that helps a little, I was confused by it for quite a while. Hell, I might still be; I'm sure someone will slap me silly if I got something wrong. Good luck with your coil building, it's pretty damned rewarding once you get the hang of it.
