How to white balance the each RGB components of an image in matlab?

You can't white balance a single color channel (that doesn't even make sense because a single channel is monochrome, not color), but if you want to make the means of each color channel the same, then you can do this

grayImage = rgb2gray(rgbImage); % Convert to gray so we can get the mean luminance.
% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
meanR = mean2(redChannel);
meanG = mean2(greenChannel);
meanB = mean2(blueChannel);
meanGray = mean2(grayImage);
% Make all channels have the same mean
redChannel = uint8(double(redChannel) * meanGray / meanR);
greenChannel = uint8(double(greenChannel) * meanGray / meanG);
blueChannel = uint8(double(bluedChannel) * meanGray / meanB);
% Recombine separate color channels into a single, true color RGB image.
rgbImage = cat(3, redChannel, greenChannel, blueChannel);