multitexturing alpha masking

OpenGL OpenGL: Basic Coding
1

I want to texture a quad so it look like this:

Having these 3 textures:

How do I combine those textures with glTexEnv? What sort of glBlend should I use?
Should these textures be rgba bitmaps or will rgb do as well?

Thanks!

2

Use shaders and forget about glTexEnv
http://www.opengl.org/wiki/index.php/Multitexture_with_GLSL