Neznajka,
так проще ...
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script>
document.addEventListener("DOMContentLoaded", function() {
let radio = document.querySelectorAll('[name="radTri"]');
let per = 1;
radio[per].checked = true;
})
</script>
</head>
<body>
<div id="infaSpsVal" class="marLft borRadBot">
<div class="optWrap">
<input id="rad1" type="radio" name="radTri" value="-1">
<label for="rad1">Papier</label>
<div class="radInd"></div>
<div class="radFon"></div>
</div>
<div class="optWrap">
<input id="rad2" type="radio" name="radTri" value="0">
<label for="rad2">-?-</label>
<div class="radInd"></div>
<div class="radFon"></div>
</div>
<div class="optWrap">
<input id="rad3" type="radio" name="radTri" value="1">
<label for="rad3">System</label>
<div class="radInd"></div>
<div class="radFon"></div>
</div>
</div>
</body>
</html>