"It is not three o'clock."
In this case ではありません is the whole ending particle. It's simply the negative form of です. NHK's easy japanese does a good job explaining these grammar points https://www.nhk.or.jp/lesson/english/teacher/4.html
When you want to express where a certain action is taking place, you use particle で. (Where can I buy an apple? => どこでりんごを買かったらいいですか。) [Where で apple を buy is か?] If you are specifying a place where something is happening, you are actually describing a place where things are, where things do exist, use particle に. (There is a bag in that room. => あの部へ屋やにカバンがあります。) [That room に bag が exists.)
Not quite... The thing most people don't like to think about is that Japanese→English is not a direct translation (which is why Google Translate struggles with Japanese).
時 is actually saying "hour." So when they say 「三時二十分です」、they're really (direct translation) saying "It is 3 hours 20 minutes."
You'll hear them say (for example) 「ここから駅は一時ぐらいです」、which is saying, "From here, the station is 1 hour." In this case, you can see how 時 is related to time, but isn't saying "o'clock."
今 does mean now... So that was right.
Its hard to explain and people dont understand. Youre getting dewa- and wa mixed up. Its like this: if you are saying the time you just say "San ji desu". No wa needed. To say the opposite you change desu to 'dewa arimasen'. Ending a normal verb in -masen indicates the negative form. (FYI desu itself is a contraction of de aru).
Wa as a particle is used to indicate subjects. So if you said The time is now 3 o'clock, then you would say , Ima wa san-ji desu. Japanese doesn't use spaces in a lot of those words so it is kind of hard to differentiate the particles and word fragments. I don't like though that DL separated arimasen into parts...