Ayane
Ayane is a very difficult match-up for Mila, but only when the Ayane player knows the match-up. She has a lot of quick tracking attacks, so you can't rely on stepping at neutral. She also has a lot of strings that end with tracking attacks, so you can't really rely on stepping the last hit of a string either. Instead, you have to either block and punish, or use 3H+K to interrupt.
After Ayane's 4P and other attacks that have a high or low kick ender, 3H+K crushes both. Now if the Ayane player delays the high kick, it will hit you, but Mila will be airborne so the damage is minimal. If you can make good reads, Ayane players like to use low punches, which give Mila a pretty decent combo if you counter them.
I generally stay away from Ayane players and turtle as much as possible. Ayane has far better spacing tools than Mila, but she's even more deadly up close. If you turtle well, the Ayane player will either whiff a lot (punish with 4H+K or 3H+K), or try to close in to fight.
If the Ayane player goes for 66KK shenanigans, I tend to either double step immediately after blocking 66KK (the timing on this is strict) to avoid BT PPPP, or I use 2H+K to go under Ayane's high options and even a few mid options.
Hitomi
I really dislike fighting Hitomi online. You can't step her low sweep and it's very hard to react to it online. It's also very difficult to counter her mid kick, string-ending options even though it's telegraphed for days. So online I SS~tackle as much as possible, and take the low sweep hits when they come. After getting hit by a sweep, I just step again. Most Hitomi players won't sweep more than twice in a row, and I make up the damage with a single tackle (in most cases).
For those Hitomi players that will just spam the sweep against Mila, I just spam 3H+K. Go over their sweeps a few times with that and they'll stop spamming it.
Offline, Hitomi is pretty much locked down by Mila. She can't use her high or low string enders because Mila can 3H+K on reaction. If 3H+K is blocked, step after it the first few times to force the Hitomi player to use 1P to stop your step. As soon as they start doing that, use back to back 3H+K.
You do need to be careful of her advanced mid kick hold as it can lead to good damage, especially if there's environmental damage to be had. Also, after Hitomi's 6T, I usually either step or use 2H+K. Stepping avoids most of Hitomi's preferred follow-ups, and 2H+K avoids another 6T.
Kasumi
This match is really stupid online. It's almost impossible to react to her low attacks at the end of her strings, and they all track and stun on NH. She can also beat you out with ridiculousness because it's online and her 11-frame mid beats out Mila's 12-frame mid when she's at disadvantage (yay for online!). You can't even punish most of her unsafe attacks because they become safe online.
What I generally do is just block. You'll take a few of the low attacks, but almost every Kasumi player I've encountered online has no idea what it means to be safe. While you can't punish much, you can punish 1PP and 9KK with Mila's 6T and then go into stun loops for as long as you can make it last.
It's a tough fight, but stun loops are your friend here because Kasumi players like to mash that attack button as much as possible. Very few of them actually utilize her quality tools (online at least).