Aye, red cable (5V) off a molex through a resistor of about 100Ω to the anode, and black cable (gnd) from the molex to the cathode is your best bet. Stick the three of them in parallel each with their own resistor.
If you put them in series the resistor will probably get too hot. They're typically rated at 0.25W, but for a typical 20mA for each LED through the single 100Ω resistor, the heat produced would be P=(60mA)^2(100Ω) = 0.36W.
Edit: Just seen the new posts. 3V is typical for an LED, 100Ω will give you about 3V from the 5V molex assuming about 20mA, which is typical for most LEDs.