How hot do High Power 1-3W LEDs get
Since you don't have a datasheet, you could figure this out empirically.  Run a known amount of current (start with very small currents to try and avoid self-heating) through each of the 3 diodes and measure the Vf and temperature (you can get close on temperature without measuring the junction since the thin dielectric of the MPCB has very low thermal impedance).  From this you can calculate a rough thermal impedance.  Repeat for various ambient temperatures (or just use self-heating to get to a different temperature) and currents.  Your results won't be exact, but you should get close to the same number across several measurements because as temperature goes up, Vf goes down (and thereby power).  Based on your application, you can then decide how much additional heat-sinking you may need.

Between the LED's thermal impedance and the MCPB, I would guess you will probably be in the 5 C/W range from past experience.