I have todo lists that are hundreds of lines long. It gets ridiculous, especially considering that I’m likely to perform the task and only cross it out much later, as the execution of the task was independent of whatever was written in the todo list.
I actually have a [personal policy of “no todo lists”], see that article for an extended explanation as to why.
Why 3? See [small numbers of items for mnemonic lists]. 3 items will fit nicely into memory. Any more than that, it gets problematic to remember, much less to perform.