Number agreement of "a thing of the past"

Is the pluralization correct in the following sentences?

1. To Do lists are a thing of the past.
2. To Do lists are things of the past.
3. A To Do list is a thing of the past.

Are they all (grammatically) correct? Which of the three is the best English? Why?

As I believe your intuition says, "are things of the past" is much more common than "are a thing of the past". Consider the Google Ngram below:

And looking at the actual citations, a reasonable fraction (at least a quarter of them) are grammatical constructions parallel to "to-do lists are things of the past".

However, Google still finds many, many examples parallel to "to-do lists are a thing of the past". These are grammatically correct because in the construction A is/are B, English makes no requirement that A and B have the same number; the requirement is that the verb agrees with A. For a different example, I think that native English speakers would be more likely to say "to-do lists are a problem" than "to-do lists are problems". You can use Ngrams to check that this is currently true (it's changed from 50 years ago) if you replace to-do lists with men, women, or people.

All three are "correct enough," but these two variations are most common:

• To do lists are a thing of the past.
• To do lists are things of the past.

Of those two, the first is most common, with 68 million examples indexed by Google. The second of those two has 18 million examples indexed by Google.

The second one is more grammatically correct than the first, as the nouns agree in number.

Your first sentence is correct because "a thing of the past" is an idiomatic expression; "things of the past" is not.

http://idioms.thefreedictionary.com/a+thing+of+the+past