function setText() {
name3.value = name1.value + name2.value;}
<input type="text" name="name3" onFocus="setText()">
name3.value = name1.value + name2.value;}
<input type="text" name="name3" onFocus="setText()">
var frm = document.forms[0];
frm. name3.value = frm.name1.value + frm.name2.value;}
<input type="text" name="name3" onFocus="getValue()">