So would a resistor with a 3R on the common cathode do the same job as a single res. of value R on each of the 3 color leads?
No. Once again, when two LED's are turned on they will have less brightness than a single one on by itself (and with 3R, they will all be less bright to begin with). With a single R in each lead the brightness will be the same regardless of how many are on. In theory...it depends on the rest of your circuit and what it's current-sourcing/sinking capacity is.
And wouldn't any configuration lead to a dimming when more than one color lights up?
No. Look at the circuit for our Gadget Shield
(second page, on the right side):http://ruggedcircuits.com/AS010/as010.pdf
This circuit keeps the same brightness in all LED's regardless of how many are on.
The Rugged Motor Driver
: two H-bridges, more power than an L298, fully protected