Her routine is (give or take half an hour):
7:00 - Wake for the day, feed
8:00 - Sleep
10:00 - Feed, take bath
11:30 - Sleep
13:00 - Feed
14:30 - Sleep
16:00 - Feed
17:30 - Sleep
19:00 - Feed, bedtime routine
20:00 - Sleep
10:30/11:00 - Dreamfeed
Like I said before we have been working on the 45 min. naps which she was taking, but now she can go 2 sleep cycles (with just a little bit of our help). This is her ideal schedule. Sometimes it will be off a bit (like today, when she woke in the morning at 9:00, and so did I !). Also, we've had it rough a few weeks ago, when I was working out how I could get her schedule to fit in with her sister's. To be honest I can't say when things went haywire from my lack of sleep, but I have this idea that when her naps went to 45 minutes all the time, that's when her night sleep went crazy too.
Ok, to answer other questions
What her weight is? At her last appointment 5,300 Kg - 11,7 Pounds
Is he bottle or breastfed? Breastfed. Both breasts at each feed.
Is she always hungry at her 3 hr feeds in the day? I think so, at least she eats well and when I'm getting ready she looks at the breast anxiously. If she's in Dh's arms she'll fuss until I get her.
I am trying to see if she is getting enough milk intake in the day, as it sounds like she is still hungry at night and waking for food??! 
At first I thought so too, so I would feed her at night, but she would wake an hour later anyway !!! So one night I stopped the night feedings and after calming her down at around 5am, I gave her another dreamfeed thinking she hadn't eaten in 7 hours, and to stop the wake-feed-sleep association. She still woke up an hour later. So I thought that it was definitely a habit, so I stopped all night feeds. This was a few nights ago. Last night, at 5:00 I was just so tired and she was sooo hard to settle (I patted for over an hour and she wouldn't sleep), I tried feeding her. She had a full feed and slept until 9:00 this morning. I CANNOT understand this baby !!!
You probably are experiencing a growth spurt at the moment - very common around 12 weeks. She had one at 11 weeks! Is she having another

?
The other things to think about are - if your DD is having some developmental milestones (could she be ready to roll already??) then those can effect night sleep. She did started sucking on her thumb very early, so I thought about this too, but I haven't seen anything yet. She is "talking" a LOT lately.
Take a really good look at how your lo falls asleep - it's great that she is falling asleep independently but if you are making her really drowsy before bed then that is still a sleep association that you have to deal with IF it leads to nightwakings. This is how it happens: when I see her fuss, rub her eyes or yawn I pick her up, take her to the room and turn on the music box. She usually starts to cry at this point, in my arms. I put her down in her cot, and as soon as she pops her finger in her mouth she stops crying, and off she goes. Sometimes she needs a little patting. If she was fussy to sleep she might jolt at the 20 min.mark, but again that's not the norm. Night time routine is different because we put on PJ's, and say goodnight to daddy and big sister. I have tried sitting with her to calm down before bed but she only cries more. I've found that letting her be in the cot earlier and letting her have her thumb gets less crying.
Could she have "cot phobia"? She has always cried before sleeping, but then would settle herself.
Maybe she is ready to move to 3.5hour Easy...that often helps fix night problems. How should I go about it? I don't think she can handle 2 hours awake time.
I've also thought that her night wakings could have different reasons each couple of nights (too tired one night, hungry the next ...) and I'm thinking she is into a pattern when there are a lot of different things going on, but the longest sleep stretch I've had in 2 weeks were the 4 hours I got last night so I'm very exhausted. I just need to understand what is happening with this baby and move on with a plan to get her sleeping at night.
Sara