I guess that the running tasks per folder recipe may help.
Update
Following the ideas in the recipe, and oversimplifying your sample just to give the idea, this can be a solution:
var gulp = require('gulp'),
path = require('path'),
merge = require('merge-stream');
var folders = ['httpdocs-site1', 'httpdocs-site2', 'httpdocs-site3'];
gulp.task('default', function(){
var tasks = folders.map(function(element){
return gulp.src(element + '/media/sass/**/*.scss', {base: element + '/media/sass'})
// ... other steps ...
.pipe(gulp.dest(element + '/media/css'));
});
return merge(tasks);
});