Otsu Thresholding for multi pixel value

Let say i have a grayscale retinal image like this which has three different pixel value which is 100 (background),130(vessel) and 70(dots). Can i only segment the 70 pixel of the dot out from the image to become a binary image by otsu thresholding method in matlab? If can, can pls provide some guidline or code? thank you for the help.