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I assume it's always a (vs)noun, so 開始三秒 must be a noun composed of 開始｛かいし｝
and 三秒｛さんびょう｝? If that is true, does it mean "before 3 seconds have elapsed?"
(it would seem so from the context), but given that 開始 usually means something
to do with beginnings I am probably wrong. This came up in this sentence:

> 二度寝｛にどね｝の誘惑｛ゆうわく｝を開始三秒でノックアウトして、姿見｛すがたみ｝の前で軽く全身をチェック。

As I understand it so far: "Within 3 seconds / before 3 seconds have elapsed
[?] I beat the temptation to go back to bed again. I then check on my body in
the mirror [without too much fuss]."

お助けください
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2015-03-17T19:51:23.863
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2015-03-17T23:22:15.730
2015-03-17T22:01:38.757
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meaning
開始三秒 - what does it mean?
Just a speaker and no expert on the grammar of this, but 開始三秒【かいしさんびょう】 means
"Three seconds after it started/Three seconds in", so your translation wasn't
far from that.

I've always assumed it's just a modern way of shortening
「開始【かいし】してから三秒【さんびょう】」.
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2015-03-17T22:57:19.847
23320
2015-03-17T22:57:19.847
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「開始三秒」, in this particular context, means " **three seconds into the match**
".

Why "match"? Because the author is using a metaphor in likening resisting
one's temptation to go back to bed to a boxing match. The person "won" the
match in just three seconds into it.

Your TL shows you have got the gist of the sentence.
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2015-03-17T23:22:15.730
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2015-03-17T23:22:15.730
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