Object.keys(a).map(k => a[k])

I usually use this kindunderscore

npm install underscore --save

var _ = require('underscore');
var a = {"1":[1,2,3], "2":[2,3,4], "3":[3,4,5]};
var values = _.values(a);
console.log(values);

First of all, the dictionary is unordered, so the obtained set is also unordered. It is the default order of the browser
You can use for in

var arr = [];
var cont = 0;
for(var i in a){
    for(var j=0; j<a[i].length;j++){
        arr[cont].push(a[i][j])
    }
    cont++;
}
console.log(arr)

There will definitely be no problem with compatibility like this