OpenGL glBlendFuncSeparate

I need some help with OpenGL textures masking. I have it working but need to find some other blending function parameters to work in other way. Now I have:

//Background 
...code...
    glBlendFunc(GL_ONE, GL_ZERO);
...code

//Mask
...code...
    glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_DST_COLOR, GL_ZERO);
...code...

//Foreground
...code
    glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA);
...code

Now it sets foreground's opacity to 0 (fills with background texture) where mask is transparent. I need it to react to mask's colors. I mean something like setting foregrounds opacity depending on mask's color. For example if mask is black (0.0,0.0,0.0) then the opacity of that place in foreground is 0 (is filled with background), and if mask is white (1.0,1.0,1.0) then the opacity of foreground is 1 (not filled with background). It can be in reverse consequence (white = opacity 0, black = opacity 1). I just need it to work depending on color.

My current result's visualization bellow.

Background:

Mask (circle is transparent):

Foreground:

Result:

And I want it to work like this:

Background:

Mask (circle is white, background is black):

Foreground:

Result:

So that later it could be used like this:

Background:

Mask (circle is white, background is black):

Foreground:

Result:

Attempt with @Gigi solution: