Yes it is very possible. You can access the Index of the current item with the DataItemIndex property.
// Since the DataItemIndex is zero based, add 1 to it before you do modulus 3 or else the 4th item will be the first to be marked. If you want the 1st item to be marked as well as every third, then don't add 1 to it.
<%# (DataItemIndex +1 %3 == 0)?”A third Record”:”Not a third record” %>