Congradulations on your new baby girl! Very common, yet very tiring for you, especially since you have a 3 year old too who likes to sleep at night and be awake during the day...opposite of your new baby girl. :) I'd try to wake her during the day to eat regularly every 3 hours at least and try to keep her awake after eating for a while. Not easy I know, but that play time after eating during the day helps her to understand, or learn the pattern, that daytime is playtime and then when she's not awakened to eat, that's night time and time for everyone to get sleep. Also, feeding during the day in the light and in the dark at night can help the baby to learn the pattern. She's only five days old, so you've got another good few weeks of night feedings, but with helping the baby to learn that pattern, everyone can sleep restfully at night sooner rather than later. :) My first son was a good sleeper naturally, my second was not such a good sleeper naturally, but still slept through the night starting when he was 2 months old because we did those things to help him get that restful sleep. My first son was a early and very small, so we started with feeding him every 2 hours and putting him down for a nap the second hour after each feeding. Then he pretty quickly progressed to eating every 3 hours and going down for a nap the second 1.5 hours of each 3 hour increment. (My second son started with a 3 hour schedule but he was quite a bit bigger than the first.) We lengthened the increments as he grew and showed signs of being ready to lengthen it. His main sign of being ready to lengthen the increment was when he was sleeping through the night and still had to be awakened for each feeding. Then we knew he was eating plenty to last longer (I nursed so didn't go by knowing how many ounces were consumed) and needing that longer stretch of nap time so we increased to a 4 hour schedule, sleeping the last 2 hours of each 4 hour increment. By then, they were doing the pattern themselves. At first, it takes a little while for them to get used to staying awake after eating and learning it all, but the pattern does help a lot. I wasn't real strict with it by any means. If the baby was hungry early, I'd feed him, but that pattern really helps to know weather they are hungry or tired (since they acted the same for either), and helps so they are not hungry and tired at the same time (often not a good combination :) )
Hope this helps!
Best wishes to you!