Powering a 5V LED strip with CR123 Batteries
As long as you select a proper current-limiting resistor to put in series with the LEDs, the voltage you choose doesn't matter... as long as it's greater than the LED threshold voltage, so I would be wary of using 3V.
You can't easily tweak the resistance that I assume is built into the light strip controller, but if you select 6V and figure that the operating current of this strip is specified to be 0.6A (from the product listing on Amazon), then in order to burn off the excess power and present 5V to your lights, you could use a R = V/I = (6V - 5V)/0.6A = 1.67Ω resistor in series with the batteries.
Note that P= I2R = (0.6A)2 ✕ 1.67Ω = 0.6W, which may be more than your average kit resistor can handle without exploding.