its simple.......
<script language=vbscript>
sub button1_onclick()
call assign(text1,text3)
end sub
sub button2_onclick()
call assign(text2,text3)
end sub
function assign(txt1,txt2)
txt1.value = txt2.value
txt2.value = "new three value"
end function
</script>
<P> </P>
<INPUT type="text" id=text1 name=text1 value=one>
<INPUT type="text" id=text2 name=text2 value=two>
<INPUT type="text" id=text3 name=text3 value=three>
<INPUT type="button" id=button1 name=button1 Value="3 - > 1">
<INPUT type="button" id=button2 name=button2 value="3 -> 2">
guess, this is what you are looking for.