You need to be sure to use the present perfect in this situation. It's an ongoing action that is still connected to the future. -- HAS/HAVE + past participle ( regular verbs = +ed/ irregular verbs = 3rd column from verb list.)

Yes, that is the meaning that would be understood in conversation, but that usage is not correct. Correct usage would be: It's been five years since they got married. It's been three years since the young man graduated from the college. [more typically, "from college," unless a specific college has already been mentioned.] In this case "it's" means "it has".

You're correct!