Nice garden!!!
I wish I have one.
Anyways, here's my thing about composition...sorry if I am talking crap.
On your third picture, you want the attention to be on the nearest flower. Ok, what about others? I mean when I take a picture, I think of my subject, but I also think of the entire picture. The flower in the middle ground towards the right is half out of the picture, which bothers me. Why? See, now I just want to look at the left 2 complete flowers, and that somehow it doesn't make sense when you consider the whole frame.
What I would have done is also include the third flower, because it helps to complete the picture. Also, it being on the bottom helps to "stablize" the weightage of the picture. If you haven't realised it yet, the 3 flowers will form a upright triangle.
No I don't think there is such a rule, I just speak as I think.... damn, my brain is malfunctioning again
If I don't want the third flower to do anything, I will probably take the picture in another angle. Let's put it this way, you don't want anything to harm your composition.