document.addEventListener('DOMContentLoaded', function () {
	var btn1 = document.getElementById('button1');
	var btn2 = document.getElementById('button2');
	var btn3 = document.getElementById('button3');

	function callback() {
		var span = this.nextElementSibling.querySelector('.counter');

		span.innerText = parseInt(span.innerText) + 1;
	}

	btn1.addEventListener('click', callback);
	btn2.addEventListener('click', callback);
	btn3.addEventListener('click', callback);
});