Congratulations on the birth of your second baby :)
You soundly key you're doing really well. You are obviously worried about the issues you faced feeding your first baby, but you sound like you're finding ways to successfully feed this one, even just three days in :)
Firstly - you absolutely can feed a baby successfully, and exclusivly, from one side if it comes to it. Your boob will produce enough milk on a demand basis, so if you feed from just one then what I taken will be replaced.
Secondly, three days is so so early. You almost certainly won't have to resort to feeding from just one side. If the sore side is too sore to feed from, try expressing a bit (either catching it to feed later or just wiping up, don't worry about it), and just feed from the non-sore side. You might well find that the boob you feed from gets enormous compared to the non-feed side, but don't worry - it'll even out if you can eventually feed from the sore side, or the difference in size will really reduce once feeding becomes established. Either way, you won't be so lopsided forever.
I was an extreme example of one-sided-feeding-in-the-early-days because I had a cracked nipple at about a week and was feeding twins. I expressed from the sore side for a week while it healed, meanwhile feeding both babies from the ok side. I did have one enormous boob for a few weeks, but after that initial week to help healing I successfully fed from both sides.
In contrast, my sister had very sore nipples when feeding her newborn. She and her baby quickly favoured one side, and she's subsequently fed him from just that side. He is now three years old and still feeding, and while the lopsided was was noticeable for a few months, mostly her noobs looked about equal.
It really feels overwhelming at the time, because we all want to do the right thing and do the best for our babies. But our bodies really are amazing - at three days the best thing to do is give lots of boob access to the baby, express gently from any really sore nipples, and try to have faith that the boob-baby relationship will somehow work things out. I know that it's hard to believe that, but if you can keep putting the baby on a boob (either boob!) then you'll be well on your way to feeding them successfully :)
Congratulations and enjoy those newborn snuggles :)