GLSL OpenGL Each Light Added Get's Darker - opengl

Question

I have a scene that works perfectly with one light. However, when I add two more - each new addition becomes dimmer until it is almost unseen. Is the attenuation factors wrong or could it be something else?
int i = 0;
for(i=0; i<3; i++){
if (lights[i].enabled == 1.0){
//Lighting Attributes
vec4 light_position = vec4(lights[i].position,1.0);
vec4 light_ambient = lights[i].ambient;
vec4 light_diffuse = lights[i].diffuse;
vec4 light_specular = lights[i].specular;
float light_att_constant = 1.0;
float light_att_linear = 0.0;
float light_att_quadratic = 0.01;
float light_shine = 1.0;
//Object Attributes
vec3 obj_position = n_vertex;
vec3 obj_normals = n_normal;
vec4 obj_color = n_colors;
//Calc Distance
vec3 distance_LO = (obj_position - light_position.xyz);
float distance = length(distance_LO);
//Normalize some attributes
vec3 n_light_position = normalize(distance_LO);
//Apply ambience
finalColor *= light_ambient * global_ambient;
//Calc Cosine of Normal and Light
float NdotL = max(dot(obj_normals, n_light_position),0.0);
//Calc Eye Vector (negated position)
vec3 eye_view = -obj_position;
//Check if Surface is facing the Light
if (NdotL > 0){
//Apply lambertian reflection
finalColor += obj_color * light_diffuse * NdotL;
//Calc the half-vector
vec3 half_vector = normalize(light_position.xyz + eye_view);
//Calc angle between normal and half-vector
//See the engine notebook for a diagram.
float NdotHV = max(dot(obj_normals, half_vector), 0.0);
//Apply Specularity
finalColor += obj_color * light_specular * pow(NdotHV, light_shine);
}
//Calc Attenuation
float attenuation = light_att_constant / ((1 + light_att_linear * distance) *
1 + light_att_quadratic * distance * distance);
//Apply Attenuation
finalColor = finalColor * attenuation;
}
}
color = vec4(finalColor.rgb, 1.0);

Answer 1

You multiply in your colours. This means that shadows will get darker.
If you have an area around some relative brightness 1/2, then you multiply it by 1/2 (contribution from that light), you'll get 1/4.
If you have Photoshop or Gimp you can test this yourself with the Multiply blending mode, and three circles, pure red, pure green and pure blue and overlap them. Compare Multiply to Linear Dodge (the plus operation in Photoshop.)
Here's an example.
You'll most certainly want an additive effect, that is, add the terms together.