I had a similar situation. Had a bunch of letters from 5ish years ago that were really good but old, and then got some newer letters as well to highlight new experiences. I didn't have the option of a "composite" letter, so no advice there. Submitted 2 old and 3 new-- covers the bases of 2 science profs and one non-science, as well as highlighting my nonacademic stuff.
No one seemed to have any issues, but I'd recommend getting the older letters updated with a sentence or two. If schools are going to nit-pick about you not using the composite letter system because you wanted to update letters, the can shove it.