Not sure if you already know this, but the best and most reliable way to drive a LED is with a constant current, rather than a constant voltage.
I'm not sure if I know that - you mean DC verse AC but I was under the impression that DC current is constant and that the only issue in my situation would be not to feed too much voltage into them. I'm a novice with electronics so feel free to teach me what I'm missing!
OK No problem.
No not DC versus AC- just DC.
The forward voltage of a LED for a given current is indeterminate and varies from sample to sample even of the same LED type and from the same batch. The forward voltage is also dependent on color, age, and temperature, especially with the earlier LEDs.
The main objective with a LED is to have it look as you like, in terms of brightness etc. But once again, there are more variables in this because the efficiency of LEDs also varies quite a bit. With early LEDs the variation in brightness is quite large as is the beam angle.
So the best way to illuminate a LED is to feed a defined DC current through it (in electronics this is called a constant current). The LED will then be able to have whatever voltage across its pins that suits it at a particular brightness and temperature.
You can generate a semi constant current by putting a resistor in series with the LED and connecting the circuit to a DC voltage, but this is not very accurate unless the voltage is relatively high. Or you can use a simple circuit to generate an accurate constant current. Note that although the current is constant, it can be set to any value that you like, within reason that is. But once set the current stays constant irrespective of the LED forward voltage and any changes in the LED forward voltage with temperature etc.
Adjusting the input voltage to be in exactly the right spot on on the diode's I/V curve would be much more complicated.
It would be harder than that.

A 3V coin cell has an internal resistance high enough to limit the current.
Afraid this is not true.
