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?