Hey!
Welcome to MATLAB!
MATLAB provides support for multilevel thresholding through the function multithresh. Here, since you need 3 regions, the number of threshold values (N) when using multithresh will be 2.
im = imread('Your Image'); % Read your Grayscale image here
levels = multithresh(im, 2); % Get two threshold value outputs to divide into three regions
segIm = imquantize(im, levels); % The output image has quantized intensity values
imshow(segIm);
"...then color them in to red green and blue."
Since your input image is a grayscale image, it's not clear to me how you want to add color to it.
You can alter the callbacks for different components in your app to include these functions. Also, as an extension, consider using App Designer to ease your process.
Hope this helps!
