If your 13 digit "number" is really text, that is you don't intend to do any math on it, you can precede it with an apostrophe
Sheet3.Range("c" & k).Value = "'" & Sheet2.Range("c" & i).Value
But I don't see how a 13 digit number would ever get past the If statement because it would always be greater than 1000. Here's an alternate version
Sub CommandClick()
Dim rCell As Range
Dim rNext As Range
For Each rCell In Sheet2.Range("C1:C30000").Cells
If rCell.Value >= 100 And rCell.Value < 1000 Then
Set rNext = Sheet3.Cells(Sheet3.Rows.Count, 1).End(xlUp).Offset(1, 0)
rNext.Resize(1, 3).Value = rCell.Offset(0, -2).Resize(1, 3).Value
End If
Next rCell
End Sub