Hi "It can," or (more commonly) "It could happen," is correct. "It could," has the sense that you were trying to create with your second sentence above.

"It can happen" is correct. Passive voice (be + past participle) is only possible with transitive verbs. "Happen" is intransitive, so "be happened" is impossible. It has no meaning.

I'm sorry but there isn't enough information in the question to give you a correct answer. The "third form" of the verb isn't a standard way of naming any conjugation that I am aware of. Possible correct forms: It can happen It can be happening It could happen It could have happened It could be happening. Incorrect: It can be happened. I hope this is helpful. If you you need more information, please provide an explanation of what you are trying to say or information about what form you are calling the "third".