3 Watt LEDs in Series - Using constant voltage source
Yes and no.  Yes, you can add a suitable resistor, calculated the same as for a single LED, just sum the forward voltages.

NOTE: CALCULATE THE REQUIRED POWER FOR THE RESISTOR - you don't want to set fire to the resistor...!

BUT

A resistor is not recommended for high wattage LEDs as the resistance changes with heat, and at that wattage there will be a lot of heat about.

You really should consider either creating a constant current source, or using constant current sinks (note: for the latter you can keep your existing supply, just add the components to each chain to limit the sink current).

For example, a couple of circuits I found on another question on this site: