An (a) is dropped in three of them. (if it was someone, we had to drop it, but here we should use it)
The first one means I am currently married. (married here is an adjective)
The second one means I married in the past, whether I am married to that person or not now.
The third one means sometime in the past I married to that person. (the difference between the second and third one is, the third one talks about a period in the past, not just the event of marriage in the past which happened. )
1. I am married to a US citizen and we are living happily.
2. I married a US citizen in 1991 and we moved to the United States.
3. I was married to a US citizen in the duration of the second world war and then we divorced.