There are standard fourth-, fifth- and sixth-order reference marks, too: they are the section mark (§), parallels (||), and number sign (#), after which the cycle repeats with doubles, triples, and so on: *, †, ‡, §, ||, #, **, ††, ‡‡, §§, ||||, ###, ***, †††, ‡‡‡, etc. Beyond three, numbered footnotes are always preferable, even if you are David Foster Wallace.