As soon as you said Day 3 I realised what was going on.
Day 3 is notorious for that hormonal drop known as the baby blues.
Many a mother can be found crying in the shower from days 3-10, because it's all too hard and you're exhausted.
What's actually happened is you've had a big hormonal shift.
Feeling this way is completely normal, your body has just lost a lot of the feel-good hormones from pregnancy and your body is now producing a lot of prolactin which helps produce milk.
You haven't failed. You're going to be okay. You're probably going to cry a lot in the next 2 weeks or so, it's completely normal.
Also if your breasts are hard, that means your milk has come in.
Keep feeding the baby, use hot face towels to soothe the breasts, also take warm showers, and hand express to relieve the discomfort. If there's any swelling in your armpits put cold compresses there.
Watch for any redness, but it will settle down after a week or so.
If you feel flu-like symptoms, or there are red patches on your breasts (or armpits, I had this), see a Dr asap as you may have developed mastitis.
Also, use lansinoh on your sore nipples. Don't use nipple shields this early, wait a few more days until your milk supply is more established.