I believe you may need to add the following line in your logic:

Application DoEvents

Not sure exaclty where!

I believe whay rinks needs is a method of entering a value in a cell and
at the same time getting the calculation  in the form  done.

I guess this is not possible in conventional form built mode but I guess
thru control toolbox mode

Doevents will not allow access to keyboard.

Sorry to be blunt, but your code  is very hard to understand.
It seems to have nested subs, some of which do nothing.

You said "as soon as I hit  Calculate or update  button", but
I don't see an calculate  or Update... I see what looks like an
"add" button, but all it does is sets the variable "ws" as
the worksheet called "data", then exits. ws is defined within the
sub, so it isn't even available outside the sub, nor is it used anywhere else.
I don't see ANY code that does any type of calculation.
The only thing I DO so is the commandbutton2 event that copies the
values to the sheet.

The answer to your question though is

Textbox3.value = textbox1.value + textbox2.value

You can also add this to the textbox_Change event to continually update
the value in textbox  3 without the need to hit a "calculate" button.

will add the contents of the first two boxes and put the result in the third
box.
If you put this in whatever click event you use for the "calculate" button,
it should display  the result in box 3...

hope this helps,

Can i send the file directly to you, if
you can see how to add codes for update  push button

Move the code  which updates the userform  on open to a separate
sub/function. Call the sub on activation and then call the sub after
you do the calculation.