First of all; Welcome to Superuser.
If you are familiar with Macros/VBA in excel, you can use the following code to get started. Sorry, this works for me, but I don't have Excel 2003 to test it before giving it to you. That means I can't guarantee it will work for you without some additional work.
Copy and paste this into a VBA module. You will need to change sheet names to be the names of your tabs.
Private Sub Workbook_Open()
Run "timer"
End Sub
Sub timer()
Application.OnTime TimeValue("08:00:00"), "Print_1"
Application.OnTime TimeValue("10:00:00"), "Print_2"
Application.OnTime TimeValue("12:00:00"), "Print_3"
End Sub
Sub Print_1()
' Prints the sheet for 8am
Sheets("sheet1").Select
ActiveWindow.SelectedSheets.PrintOut Copies:=1
End Sub
Sub Print_2()
' Prints the sheet for 10am
Sheets("sheet2").Select
ActiveWindow.SelectedSheets.PrintOut Copies:=1
End Sub
Sub Print_3()
' Prints the sheet for 12pm
Sheets("sheet3").Select
ActiveWindow.SelectedSheets.PrintOut Copies:=1
End Sub
Hope this helps you.
Note: The system uses a 24 hour clock. So in order to use times after noon, add 12 to it. For instance, 3 pm is 15:00:00.