There's probably a clever formula to do this but here's a macro:
Dim StartDate As Date
Dim i, j, k
StartDate = "21/1/2007"
k = 1
For i = 1 To 90 ' change 90 to suit
For j = 1 To 3
Cells(k, 1).Value = StartDate + i
k = k + 1
To use this, press ALT + F11 to open the Visual Basic Editor. Insert > Module. Then copy the code and paste it in. Close the VBE. Tools > Macro > Macros, click on DateSeries and click Run.
Note: this will over-write whatever is in column A.