One could use a number of different symbols, but it is common to use either a system of the same symbol in single, double, and triplicate to denote each annotation or to use a set of different symbols to avoid duplicate and triplicate notation.
For example, to use the same symbol, but doubling and tripling, you would use * for the first annotation, ** for the second, and *** for the third.
For three different symbols, you could use * for the first, + for the second, and ^ for the third.
This question, however, makes me wonder if you mean foot or end note symbols instead of annotation symbols. The above would be used for footnotes or endnotes in a formal text and the choice to use one over the other would have to be made in consideration of total number of annotations in the text. Too many different symbols could become confusing quickly.
Select one system and stick to it throughout the material being annotated.