Alternatives using 'whoever'

[Sallyann]

Which is correct?

1. Whoever leaves last has to check that the windows are locked.

2. Whoever the last person to leave is, they have to check that the windows are locked.

3. Whoever is the last person to leave, has to check that the windows are locked.

These have been offered to me as possibilities and I need to justify my choice which is statement 1.
Any comments as to the grammatical correctness of these statements will be very welcome.
Thank you.

[Mister Micawber]

.
If you remove the comma from #3, all are grammatically correct. The awkwardness with #2 is the perceived discord between the singular 'person' and the plural pronoun 'they'. The awkwardness with #3 is the juxtaposition of the two verbs 'leave' and 'has'.
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Native English teacher at Mister Micawber's

[Sallyann]

Thanks very much.

Yes I checked a few other references to the use of 'whoever' and agree that 2. could be improved by replacing 'they have' with 'he or she has'. Your reply seems in accordance with that.

In case it helps anyone, here is the other reference:
http://www.englishforums.com/English/WhoeverSingularOrPlural/zmqwm/post.htm#484261