Connecting a single LED to Motherboard CHA Fan Header
Forward drops of red LEDs are about 1.7-2.5V, with the high output ones being towards the higher end, but there's some information you're lacking here. The page says that there are 3 LEDs on the assembly, but doesn't indicate whether they're connected in series or parallel. A 50mA draw for a high brightness LED would be reasonable to indicate that they are in series, which would be logical for a supply of sufficiently high voltage to allow that.
  Fortunately, these assemblies have a series resistor built in (or at least the ones I have used have), to allow them to run on the stated voltage range, so you don't need to add any external resistor.