Wild mallard nest

Hi guys, I was looking to get some information about duck nesting behavior since I know very little about it.

A mallard duck laid eggs under a shrub in my front yard and began sitting on her nest about two weeks ago. In the very beginning the male would show up occasionally, which I read was normal for the first few days and I haven't seen him in awhile. A few days ago I noticed that two other ducks have been visiting the female and here nest in the early morning for the past three days. Originally I thought they were both female, but noticed one of them had a small green patch on the back of it's head, so I assume one is actually a male who is molting. Is this normal for a new female and male duck to visit a nesting mother or is it possible that the new ducks are trying to make a nest under the same shrub (I didn't look to see if there was a second nest being built, because I do not want to disturb the existing nest and the mother duck)? I wasn't sure if mallards are territorial and want to make sure that they aren't trying to scare her off her nest.
Any information would be appreciated!  

Sounds like the hen found the perfect place to nest and took advantage of it.
It is probable that the "visiting" ducks are part of her flock and just checking on her, if anything else was going on, you would hear the ruckus and see the fight or the evidence of the fight.
Momma ducks are similar to momma chickens when it comes to protecting their nest and babies.

Drakes are only territorial about their hens, they will protect them for mating purposes from other drakes.
But they won't chase off other breeds from an area that I've ever seen.

If she is sitting, the drakes will check up on her but they won't try to breed with her again until next year.