Hola Sasuke.
Prueba esto:
Código PHP:
<html>
<head>
<script language="JavaScript">
function agrega(cLetra) {
document.frm.texto.value += cLetra;
}
</script>
</head>
<body>
<form name="frm">
<input type="text" name="texto"><br>
<input type="button" name="btA" value="A" onClick="agrega('A')">
<input type="button" name="btB" value="B" onClick="agrega('B')">
<input type="button" name="btC" value="C" onClick="agrega('C')">
</form>
</body>
</html>
Saludos.