I think it all boils down to the point of reference taken into account: while (1) states that the speaker's 18th birthday was his/her worst day up to then (the speaker may have had worse experiences after that), (2) and (3) state that that was his/her worst day up to the present (i.e. the moment of speaking).
The difference between (2) and (3) is that (3) is the only possible form in BrE, while (2) is accepted in AmE (see the use of "ever" in this post).
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