How can I combine 3 bitmaps? However, it should be each bitmap for each color channel, RGB

Look at the BitmapData's copyChannel function.

bitmap1 will supply the red for the final bitmap, bitmap2 will supply the green value for the final bitmap etc?

if so, loop through the final bitmaps pixels and assign values using setColor() or setColor32 and the bitwise operators to extract and add rgb values.