Trzy guziki jeden licznik

    let buttons = document.querySelectorAll("button");
    Array.from(buttons).forEach(function(button) {
        button.addEventListener("click", function() {
            button.nextElementSibling.firstElementChild.innerHTML = parseInt(button.nextElementSibling.firstElementChild.innerHTML) + 1;
        })
    })

    <button id="button1">Guzik 1</button>
    <button id="button2">Guzik 2</button>
    <button id="button3">Guzik 3</button>
    <p>
        Wartość licznika: <span class="counter">0</span>
    </p>

let buttons = document.querySelectorAll("button");
let counter = document.querySelector(".counter");
let a = 0;
    Array.from(buttons).forEach(function(button) {
        button.addEventListener("click", function() {
        		a++;
            counter.innerHTML = a;
        })
    })