I think the image, as an image looks like a representation of a flag waving in the wind. But when you start think of it in 3D, it really doesn’t make any sense.
Your best option in my opinion is to use a cloth simulation as mentioned.
or
deciding if the flag is of a triangle or rectangle shape, subdivide a plane of the chosen shape and use proportional editing to move it around to the desired position.
If we assume the flag is rectangle, with 3 horizontal stripes, white, red and blue, as @spikeyxxx has demonstrated, it only looks acceptable from one angle.
