The simplest way I can think of is to concatenate the three variables as individual RGB values and display as an image like this:
% Some fake data:
[lon,lat] = meshgrid(-179:2:179,89.5:-1:-89.5);
temp = mat2gray(peaks(180)); % mat2gray normalizes 0 to 1
sun = cosd(lat);
water = abs(sind(lon));
% Optional: mask out the ocean:
ocean = ~landmask(lat,lon);
temp(ocean) = NaN;
sun(ocean) = NaN;
water(ocean) = NaN;
% Concatenate 3 variables as rgb:
im = cat(3,temp,sun,water);
% Plot:
worldmap('world')
geoshow(lat,lon,im)
