For 一代, I remember it as “generation” rather than lifetime, I know wanikani gives them both as synonyms. 代 is used in other words involving times or periods so I feel like “generation” just fits better that way.
一生 since it is the kanji for “one” and “life” I remember it as “a lifetime” or “one existence” rather than something like a generation or age as wanikani also lists.
Maybe thinking of them like that will help distinguish the two.
As for 一体, I have no tips other than what ApathyAunt said, you end up hearing it A LOT in anime / manga / dramas; if you can imagine a character exclaiming it, it has more of a harsh sound than the other two.