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From http://gamedevelopment.tutsplus.com/articles/use-tri-planar-texture-mapping-for-better-terrain--gamedev-13821

Vertex Shader

varying vec4 vPos;
varying vec3 vNormal;

void main(){

    ...
    vPos =  vec4( gl_Vertex.xyz, 1.0 );
    vNormal = normalize(gl_NormalMatrix * gl_Normal);
    ...
}

Fragment Shader

uniform sampler2D tNormal;
varying vec3 vNormal;
varying vec3 vEye;

vec3 getTriPlanarBlend(vec3 _wNorm){
    // in wNorm is the world-space normal of the fragment
    vec3 blending = abs( _wNorm );
    blending = normalize(max(blending, 0.00001)); // Force weights to sum to 1.0
    float b = (blending.x + blending.y + blending.z);
    blending /= vec3(b, b, b);
    return blending;
}

void main(){
    vec3 blending = getTriPlanarBlend(vNormal);
    vec3 xaxis = texture2D( tNormal, vPos.yz * normalRepeat).rgb;
    vec3 yaxis = texture2D( tNormal, vPos.xz * normalRepeat).rgb;
    vec3 zaxis = texture2D( tNormal, vPos.xy * normalRepeat).rgb;
    vec3 normalTex = xaxis * blending.x + xaxis * blending.y + zaxis * blending.z;
    normalTex = normalTex * 2.0 - 1.0;
    normalTex.xy *= normalScale;
    normalTex = normalize( normalTex );

    vec3 T = vec3(0.,1.,0.);
    vec3 BT = normalize( cross( vNormal, T ) * 1.0 );

    mat3 tsb = mat3( normalize( T ), normalize( BT ), normalize( vNormal ) );
    vec3 N = tsb * normalTex;
}