The passive voice sentences contain the verb to be in any tense and the past participle of the main verb.
First and third sentences follow this rule.
In the first verb to be is in Past Simple tense (was), so this is the tense of the whole sentence.
The last sentence contains the verb to be in the Future Simple tense (will be) so it is the Future Simple Passive Voice sentence.
The second sentence does not contain Past Participle, so it is an active voice sentence. The geround form studying indicates a Continuous tense. The present form of to be indicates Present Continuous tense.