Hi,
the average forward voltage range for standard LED’s is around 1.7 to 2.5 Volts and the average maximum current around 20mA , depending on the color. Some high intensity LED's require up to 4 Volts. At 3 Volt only, I recommend to run single LED's in parallel and proper resistors in series as current limiters. The resistor value depends on the forward voltage of your LED, a good start would be 100 Ohm. Nevertheless, you might be able to find LED's working at 1.5V. In this case, you could connect 2 LED's in series, but they might not draw enough current (equals brightness) for your application. Be careful when you run tests, the average maximum current per LED is around 20mA. Any higher current will shorten the life span or kill it immediately. For example, connecting a 6V battery to a LED without a resistor in series will destroy her almost immediately. Furthermore you have to watch that the LED is connected to the proper terminals (longer leg to plus).
Regards
Wilhelm