Blending RGB LED colors together
A clear LED TIR lens will preserve the angular properties of the emitter through the optical system. In an RGB LED, all 3 colors can't be dead center at the same time. Two or all three will be off center. In effect, the lens will take the 3 colors and project "an image of them" up on the wall.

There are two ways to minimize this effect. One is to place the emitters as close together as possible. It sounds like you have a multi-die emitter with all 3 LEDs on the same chip. You can't get them any closer together than that.

The other is to diffuse the light, thereby mixing the colors. You are bound to lose some lumens when you diffuse the light.

There are a number of color-mixing LED lenses available from various manufacturers. For the best result, you need a lens that is designed specifically for the LED you are using. There isn't really any "standard 3W RGB LED" when it comes to optics.
Here is one example of an optic designed for a specific RGB LED: http://search.digikey.com/us/en/products/CA10944_RGBX-MC-M/711-1146-ND/2354660