Simultaneous Tasks in Gulp

I could not understand exactly what you meant here:

  

[...] but the server task prevails over the others

Could you give an example of what is currently happening?

But, as I understand it, you want to save the sss file when saving the sss and the page is updated automatically with BrowserSync.

My default gulpfile does this perfectly, see if it works as you expect:

gulpfile.js

var gulp = require('gulp'),
    sass = require('gulp-sass'),
    prefix = require('gulp-autoprefixer'),
    minifycss = require('gulp-minify-css'),
    notify = require('gulp-notify'),
    rename = require('gulp-rename');


var browserSync = require('browser-sync').create();
var reload = browserSync.reload;


// Static Server + watching scss/html files
gulp.task('serve', ['compileStyles'], function() {

    browserSync.init({
        server: "./"
    });

    var watcher = gulp.watch(["sass/*.{sass,scss}", "scss/**/*.{sass,scss}"], ['compileStyles']);
    gulp.watch("*.html").on('change', browserSync.reload);
    gulp.watch("js/*.js").on('change', browserSync.reload);
    gulp.watch("css/*.css").on('change', browserSync.reload);


    watcher.on('change', function(event) {
        console.log('SASS/SCSS File ' + event.path + ' was ' + event.type + ', running tasks...');
    });
});

function processSass(gulp, file) {
    gulp.src('sass/' + file + '.scss')
        .pipe(sass().on('error', sass.logError))
        .pipe(rename(file + '.css'))
        .pipe(gulp.dest('css/'))
        .pipe(prefix('last 3 version'));
    //.pipe(minifycss());
}

function processScss(gulp, file) {
    gulp.src('scss/' + file + '.scss')
        .pipe(sass().on('error', sass.logError))
        .pipe(rename(file + '.css'))
        .pipe(gulp.dest('css/'))
        .pipe(prefix('last 3 version'));
    //.pipe(minifycss());
}

gulp.task('compileStyles', function() {

    processSass(gulp, 'style');
    processSass(gulp, 'fonts');
    processScss(gulp, 'bootstrap/bootstrap');

    gulp.src('./')
        .pipe(notify("Arquivos Atualizados com sucesso!"));

    browserSync.reload();
});

gulp.task('default', ['serve']);

I solved the problem by joining the watch task with sass and server , so just use the npm run gulp watch on the terminal, that all 3 tasks will run simultaneously:

var gulp = require('gulp')
,sass = require('gulp-sass')
,watch = require('gulp-watch')
,browserSync = require ('browser-sync');

gulp.task('sass', function () {
   	 return gulp.src('src/sass/*.scss')
        .pipe(sass({outputStyle: 'compressed'}).on('error', sass.logError))
        .pipe(gulp.dest('src/css'));
	});

gulp.task('server',function(){
	browserSync.init({ 
		server: {
			baseDir :'src'
		}		
	});
	gulp.watch('src/**/*').on('change', browserSync.reload);
});

gulp.task('watch', ['sass', 'server'], function () {  
    gulp.watch('src/sass/*.scss', ['sass']);
});