Design pattern for loading multiple dependencies?

The Problem

Given a number of asynchronously loaded dependencies, I want to trigger some code only after all dependencies are finished loading. As a simple example, consider the following pseudo-code:

bool firstLoaded = false, secondLoaded = false, thirdLoaded = false;

function loadResourceOne() {
    // Asynchronously, or in a new thread:
    HTTPDownload("one.txt");
    firstLoaded = true;
    if (secondLoaded && thirdLoaded) {
        allLoaded();
    }
}

function loadResourceTwo() {
    // Asynchronously, or in a new thread:
    HTTPDownload("two.txt");
    secondLoaded = true;
    if (firstLoaded && thirdLoaded) {
        allLoaded();
    }
}

function loadResourceThree() {
    // Asynchronously, or in a new thread:
    HTTPDownload("three.txt");
    thirdLoaded = true;
    if (firstLoaded && secondLoaded) {
        allLoaded();
    }
}

function allLoaded() {
    Log("Done!");
}

/* async */ loadResourceOne();
/* async */ loadResourceTwo();
/* async */ loadResourceThree();

What I'm Looking For

This is a problem that I've found myself having to solve repeatedly in different languages and in different contexts. However every time I find myself using the tools provided by the language to hack together some simple solution, like returning each asynchronous resource as a Promise in JavaScript then using Promise.all() -- or loading each resource in its own thread in Python then using threads.join()

I'm trying to find a design pattern that solves this problem in the general case. The best solution should meet two criteria:

1. Can be applied to any language that supports asynchronous operations
2. Minimizes repetition of code (note that in my simple example the line allLoaded(); was repeated three times, and the if statement preceding it was practically repeated, and wouldn't scale well if I need a fourth or fifth dependency)
3. Runs the final callback as soon as possible when all resources are loaded -- this one is hopefully obvious, but solutions like "check that all are loaded every 5 seconds" aren't acceptable

I tried flipping through the index of the Gang of Four's Design Patterns, but the few pattern names that jumped out at me as possible leads turned out to be unrelated.