vba - Implementing 3 different Macros on Multiple Cells in a Sheet by Selection

I am a complete noob at both excel and Vba and am trying to come up with code that can give me a timer and a cell color change that occurs when I click on my three buttons StartBtn, StopBtn and ResetBtn. Right now I got the code to work for a single cell B3 but need the buttons to work for any cell on the sheet that is clicked on. I am pasting the code below and hope you guys would be kind enough to go through the code and save this confused noob.

Code:

Dim StopTimer           As Boolean
Dim SchdTime            As Date
Dim Etime               As Date
Const OneSec            As Date = 1 / 86400#

 Private Sub StartBtn_Click()
   Range("B3").Interior.ColorIndex = 6
   StopTimer = False
   SchdTime = Now()
   [B3].Value = Format(Etime, "hh:mm:ss")
   Application.OnTime SchdTime + OneSec, "Sheet1.NextTick"
End Sub

Private Sub ResetBtn_Click()
    Range("B3").Interior.ColorIndex = -4142

    StopTimer = True
    Etime = 0
    [B3].Value = "00:00:00"
End Sub


Private Sub StopBtn_Click()
    Range("B3").Interior.ColorIndex = 4
    StopTimer = True
    Beep
End Sub

Sub NextTick()
   If StopTimer Then
      'Don't reschedule update
   Else
      [B3].Value = Format(Etime, "hh:mm:ss")
      SchdTime = SchdTime + OneSec
      Application.OnTime SchdTime, "Sheet1.NextTick"
      Etime = Etime + OneSec
   End If

End Sub