JSFiddle - React, Tailwind, and code Playground

HTML

<h3>Geht</h3>
<form name="formular">
<input type="text" id="Wert1"  name="1" value="" onkeyup="bp_calculate()"/ >
<input type="text" id="Wert2"  name="2" value="" onkeyup="bp_calculate()"/ >
<input type="text" id="Wert3"  name="3" value="" onkeyup="bp_calculate()"/ >

<input type="text" id="Wert4"  value="" readonly>
</form>

<h3>Geht NICHT</h3>
<form name="formular_n">
<input type="text" id="Wert1n"  name="1" value="" onkeyup="bp_calculate_no()"/ >
<input type="text" id="Wert2n"  name="2" value="" onkeyup="bp_calculate_no()"/ >
<input type="text" id="Wert3n"  name="3" value="" onkeyup="bp_calculate_no()"/ >
<input type="text" id="Wert6n"  value="" readonly>
</form>

JavaScript

function bp_calculate() {
	var options = {
	style: "currency",
	minimumFractionDigits: 0, 
	currency: "EUR"
	};


	let summe_1 = 	getInputAsFloat(document.formular.Wert1 || 0) +
						getInputAsFloat(document.formular.Wert2 || 0) +
						getInputAsFloat(document.formular.Wert3 || 0); 
						;
	
	document.getElementById("Wert4").value= (summe_1).toLocaleString("de-DE", options);

}

function bp_calculate_no() {
	var options = {
	style: "currency",
	minimumFractionDigits: 0, 
	currency: "EUR"
	};


	let summe_1 = 	getInputAsFloat(document.formular_n.Wert1n || 0) +
						getInputAsFloat(document.formular_n.Wert2n || 0) +
						getInputAsFloat(document.formular_n.Wert3n || 0) + 
            getInputAsFloat(document.formular_n.Wert4n || 0) +
            getInputAsFloat(document.formular_n.Wert5n || 0)
						;
	
	document.getElementById("Wert6n").value= (summe_1).toLocaleString("de-DE", options);

}

function getInputAsFloat(element) {
  return parseFloat(element.value.replace(/[^0-9 ]/g, "")) || 0;
}