1) "Junbi" (じゅんび）（準備) is better translated as "preparation".
2) "Ii" is in this case (as "ii" often is) used in a different way (close to kekkou, if you know that expression, in this case meaning "not needed").
All 3 seem like sentences that are expressed verbally, and, it is unclear if they are questions or statements/orders as you can't hear the intonations.
Regardless which one (question/statement), the latter 2 are more or less interchangeable, although the 2nd one may sound a bit awkward when used in the sence "I'm done with my preparations".
The first one, if not "having a ?" most often would mean "No need (for you) to prepare". It could also (without "?") be a response to someone asking "Junbi wa dekita?", in which case the meaning would be [possibly sounding a bit rude] "I ain't gonna do any preparations". As a question "Junbi wa ii?" would typically mean "So, I don't need to prepare?", but it could vary depending on the circumstance.