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PSNR is Peak signal to noise ratio and is generally used to measure the quality of reconstruction of lossy compression codecs (e.g., for image compression). The signal in this case is the original data, and the noise is the error introduced by compression.

where I(i,j) is the pixel value of uncompressed image and K(i,j) is the pixel value of image after noise has been introduced. The smaller the value of MSE, the better the compression method is.

where MAX is the maximum pixel value possible. If the image is represented using 8 bits, the maximum value is 255.

PSNR is an approximation to human perception of reconstruction quality.Clearly, lesser the MSE value, better the PSNR value will be. In other words, higher the PSNR Value, the better the compression technique is.

In your case, image 3 is the image most nearer to the original image as PSNR value is high and the MSE value is smallest among all others.