Unless you're coding in NET, you're missing some Set functions.

t = Target
should be
Set t = Target

r = Range("A:C")
should be
Set r = Range("A:C")

i = Intersect(t,r)
should be
Set i = Intersect(t,r)

Next, you need to get the column number of i in your Select statement.

Select Case i
should be
Select Case i.Column

Beyond that, your code looks reasonably good. You'll want to disable events
before your code makes any changes to the worksheet.

Application.EnableEvents = False
' code
Application.EnableEvents = True

Target.Column gives you the column number of the first column of the range
the user modified data.

Most times, user will edit one cell at a time, but be careful for those
times when users may edit multiple cells at a time such as when pressing
"Ctrl-Enter" (Enter the same data/formula into all selected cells),
"Alt-Ctrl-Enter" (Enter an array formula into all selected cells), "Ctrl-D"
(FillDown within selected cells, though from the row above, if only one row
is selected), Ctrl-Insert (Paste into selected cells), and so on.

For this reason, you may need to use

Target.Columns.Count to know how many different columns were editted at one
time with Target.Column being the left most column editted.

There may even be one of those rare cases of multiple different ranges
editted at a time, thus would need to use:

Target.Areas.Count

to see if just one range was used or if multiple ranges were used at the
time of edited cells.

Target.Areas(I)

The above is a range object, which you can use for the same set of
properties as on a single range.

If you need to set a range to a range object, just as shown in the
Sheet_Change event, you can use the following lines to declare, then
initialize.

Dim rngCurrent as Excel.Range, I as Long

For I = 1 to Target.Areas.Count Step 1
   Set rngCurrent = Target.Areas(I)
   <Perform your statements as needed>
Next I
Set rngCurrent = Nothing

As you can see, once an object variable is no longer in use, it's best to
reset it back to nothing.