All three sentences are acceptable. Though Sentence 2 is preferred in common usage to Sentence 3, they convey the same essential meaning. I will focus on the differences between Sentences 1 and 2.
"Till" (or "until") is time-related and is roughly equivalent in meaning to "up to/ending at a (stated) point in time." In a conditional or negative statement (i.e. "not until"), it can be substituted by "(only) when/after" and implies the speaker's expectation or relative certainty that a necessary action will be performed (or a necessary event will occur) at some time in the future.
Breakfast will be served until 10:00 a.m. (unconditional, simply time-related)
The room will not be available until it has been cleaned. (negative/conditional, time-related) -->The room will (only) be available after it has been cleaned.
"Not...unless" is more "purely" conditional (not implying any relationship to time) and its meaning can be substituted by "only if." The negative element of the sentence is either expressed in the clause containing the consequence (result) or implied in the clause containing the condition (requirement). Compared with "until," the speaker is showing uncertainty that a necessary action will be performed (or a necessary event will occur).
I will not go to the party unless you do. (negative/conditional) --> I will go to the party only if you do.
I will throw away the milk unless you ask me to save it. (implied negative/conditional) --> I will throw away the milk if you do not ask me to save it.