Using red, yellow and blue can be cleverly done by using tints, tones and shades. Blending the colours will create some murky colours but the scheme will be better off for it. These colours are there to make the others shine. It also adds a lot of depth to the palette.
Two copies of a photo of leaves were used to create the above image. I did this to get a real mix of the colours, giving me a greater variety to choose from. One photo was set at half transparency to achieve the effect. I find this is a wonderful technique to get a colour scheme.
While the green colour (fourth from the right) may look like a fish out of water, you can see how important and unobtrusive it is the leaves above. You may also wish to choose one main colour and the other two for highlights so they are not fighting for attention. Popping a neutral colour like the charcoal between colours will frame them nicely and give the eye somewhere to rest. Use different values of each colour to add variety to the scheme.
What can you do with this scheme?