Blending 3 textures using a shader

I want to create a shader that enables 3 texture blending, I have looked at the following code: http://www.unifycommunity.com/wiki/index.php?title=Blend_2_Textures and I’m trying to extend it, my pseudocode looks as follows:

if blending is > 0
    texture2 ConstantColor(0, 0, 0, [_Blend])
else if blending is < 0
    texture3 ConstantColor(0, 0, 0, -[_Blend])

My shader looks like this:

Shader "My First Shader" {

Properties {
    _Blend ("Blend", Range (-1, 1) ) = 0
    _MainTex ("Texture 1", 2D) = ""
    _Texture2 ("Texture 2", 2D) = ""
    _Texture3 ("Texture 3", 2D) = ""
}

SubShader {
    Pass {
        SetTexture[_MainTex]

        if([_Blend] > 0)		//<-- Parse error: syntax error at line 14
        {
            SetTexture[_Texture2]
            {
                ConstantColor (0,0,0, [_Blend])
                Combine texture Lerp(constant) previous
            }
        }
        else if([_Blend] < 0)
        {
            SetTexture[_Texture3]
            {
                ConstantColor (0,0,0, -[_Blend])
                Combine texture Lerp(constant) previous
            }
        }
    }
}

}

When I try to use it my object looks pink and Unity throws the following error

Parse error: syntax error at line 14

How can I access _[Blend] value on a conditional statement?

I don’t believe you can use conditionals in a shaderlab program like you are attempting to do here. You would need to make a pass that includes a CG program.