Number 1 would be the way I would negate this sentence. However, I would simplify even more by saying " I did not tell everyone everything about the event."

2 Does not negate the sentence. 2 could be true while the original statement is also true.

I can see how 3 negates the original in that 3 and the original statement could not be true at the same time. It just comes at the negation at an odd angle compared to number 1.