express.js with MySQL

Here is an answer following my comment since you mentioned you couldn't figure it out on your own.

First snippet uses promises, a quick helper function, but no external library. Second snippet uses the external async.js library and is a bit more callback-heavy. Both of them tackle the problem assuming we want the queries to be executed in parallel.

With promises

router.get('/', async function(req, res, next) {
    var queries = ['SELECT * FROM `test`', 
                   'SELECT * FROM `test2`', 
                   'SELECT * FROM `test3`'];

    var allResults  = [];
    /*transform our `query` array into an array of promises, then 
      await the parallel resolution of all the promises*/
    var allQueryRows = await Promise.all(queries.map(query => promiseQuery(query))); 

    /*'allQueryRows' is an array of rows, so we push each of those
       into our results*/
    allQueryRows.forEach(function(rows){
        allResults.push(...rows);
    });

    res.render('index', {
        data: allResults
    })
});

function promiseQuery(sqlQuery){
    return new Promise((resolve, reject) => {
        connection.query(sqlQuery, function(err, rows, field){
            if(err) 
                return reject(err);
            resolve(rows);
        })
    })
}

With callbacks and async.js

const async = require('async');

router.get('/', function(req, res, next) {
    var queries = ['SELECT * FROM `test`', 
                   'SELECT * FROM `test2`', 
                   'SELECT * FROM `test3`'];

    var allResults  = [];

    async.each(queries, function(sqlQuery, callback){
        connection.query(sqlQuery, function(err, rows, field){
            if(err)
                throw err;
            allResults.push(...rows);
            callback();
        });
    }, function(){
        res.render('index', {
            data: allResults
        });     
    });
});