I think Iris would be best in the mn spot as to not be too matchy with sibs.
Or you could use both names: _______-Mae Iris
Anna-Mae Iris (my personal favorite)
I think Iris and Violet are fine together, but are you planning on having more kids? If you are, and you get another girl, would you continue the flower theme? I would just plan ahead about that before definitely deciding on Iris.
Mae is adorable and so unexpected! I second the above suggestions of Mae Charlotte and Mae Alice.
Mae Vivienne (beautiful!)
And if you wanted to use Mae as a nickname, there are lots of options. Maisie, Maeve/Maeva, Madeleine (So much better than Maddie!), and tons more. There was a character in a movie played by Anjelica Huston named Maerose.
I'm unclear whether Mae and Iris are your choices for first names or middle names?
I love Mae and Iris. They're both gorgeous, but, if I already had a daughter named Iris, I'd rule out Iris as a second daughter's name. Too matchy. So, that leaves you with Mae, which is great because Mae is lovely and matches well with Violet! I like a previous poster's suggestion of Mae Vivienne. It's beautiful.