Hi Adam,

You can use a spinner to do that.
Go to View>Toolbars>Forms
On the forms toolbar select the spinner (it's an up and down broad arrow
separated by a dash)
Your cursor will now be a small cross, just click and drag where you want
the button.
Right click on the button and goto Format Control where you can set
the cell link ($B$3 in your case), you can also set the incremental value as
well as many other parameters.
To adjust the size and location of your button you need to right click
on it and ignore the flyout menu, just click on the handles and drag it
around as you want.

HTH
Martin

I put a button from the Forms toolbar (not control toolbox toolbar) on the
worksheet and assigned it this macro:

Option Explicit
Sub testme()
   With ActiveSheet
       With .Range("B3")
           If IsNumeric(.Value) Then
               .Value = .Value + 1
           Else
               Beep
           End If
       End With
   End With
End Sub

If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm