Help with adding a normalmap to a shader

Hello,

I am working with several artists on a game and we need to use real time texture blending in our game. We have been working with the following shader to achieve that affect:

 Shader "Custom/ThreeTextures" {
     Properties {
             _Color ("Color Tint (A = Opacity)", Color) = (1,1,1,1)
             _Blend1 ("Blend", Range (0, 1)) = 0.0
             _Blend2 ("Blend", Range (0, 1)) = 0.0
             _MainTex1 ("Base (RGB)", 2D) = "red"
             _MainTex2 ("Base (RGB)", 2D) = "blue"
             _MainTex3 ("Base (RGB)", 2D) = "green"
         }
  
 SubShader {
             Pass {
                 Lighting On  
                 ZWrite On
  
                 Material {
                     Diffuse [_Color]
                     Ambient [_Color]
                 }
  
                 SetTexture [_MainTex1]{}
  
                 SetTexture [_MainTex2] {
                     constantColor (0, 0, 0, [_Blend1])
                     combine texture lerp (constant) previous 
                     }
  
  
  
  
                 SetTexture [_MainTex3] {
                     constantColor (0, 0, 0, [_Blend2])
                     combine texture lerp (constant) previous
                     }
  
  
             }
         } 
     FallBack "Diffuse"
 }

Each of the three textures used on each object would use the same normalmap, but I've been working on this for weeks and I can't understand how to combine the above shader with a surface shader. My artists are mad at me because their textures don't look quite right without the normalmaps. Any help would be greatly appreciated, even if you can just point me in the general right direction of accomplishing this. Thanks!