Zinc oxide cream from Amazon - and can also get a white and purple tube of metanium from Amazon. My little one is extra sensitive (she’s also 3 weeks) and was so sore at 1 week with literally bright red/purple nappy rash - Midwife suggested the zinc oxide cream as it’s also a good barrier and she was literally back to normal by the next evening! Healed so nicely so now I use that overnight and metanium during the day! It’s about £4 and is a grey and white tube
The best thing you can do is give her a bath and leave her with a nappy off as much as possible and I have always loved sudocream used for all 3 of mine.