So I just got dark souls 3 (first actual souls game for me) after playing Bllodborne for a bit, I chose the knight class. In BB I focused mainly on HP and Strength, what should I level up in DS3 if I plan on playing aggressively like in BB? In BB you could get health back by attacking enemies, and my play style focused a lot on that, and my endurance in BB was never a problem, DS3 seems to be a different beast. Im worried I'll die too quickly in Ds3 if I level up "wrong." I'd like my guy to be strong and tough since I'm early in the game. On my first level up visit I increased HP+2 and Strength+1, should I focus on this two mainly, and every other level on Endurance? Or am I screwing myself doing that?
That sounds fine, is a good idea to focus on HP at first until you have a decent amount of it. If you go for a pure STR route level endurance as well, the stamina consumption in this game is way higher than in BB, specially if you use heavy weapon.
In BB in seemed that your starting weapon was viable through the entire game, with upgrades, is this also the case in DS3? Should I focus on upgrading my starting sword?
The long sword is perfectly fine, but I always found it a bit boring. But you shouldn't be afraid of leveling the weapon for now. You will end getting lots of material upgrades.
Any good early game items (mainly weapons and armor) that I should pursue? Gems or weapon upgrades?
Not really, your knight armor is fine for now and in terms of weapons the long sword is also good enough, unless you want a specific type of weapon, I'd say don't worry.
In BB I spent a lot of time grinding in order to level up before facing bosses, I was always about 5 levels above the "recommended" level when facing a boss. Should I do this in DS3 as well? It worked well for me in BB, I was pretty powerful and rarely died.
Sure why not, but at some point you'll have a hard time leveling up until you get to the next area. Don't waste too much time, a couple of levels more or less is not gonna make a difference.
Is 3 Estes flasks the most I can carry? This has me nervous as fuck.
No, you find estus shards that upgrade the number of uses (search well!), once you get one go talk to the Blacksmith
Do enemies respawn when I heal at a bonfire? Or just when I go to the fire church?
When you rest on a bonfire
Any advice I should keep in mind? There seems to be a lot more items, options, menus, etc compared to BB. Not sure what I should be focusing on atm.
If you want to keep it simple just focus on either STR or DEX, you can level both and access most weapons too. One thing you should really keep in mind, is equip load, which don't exist in BB. Basically once you go past 70% you start fat rolling, which is slower and has less i-frames. You want to keep that number below 70%. Depending of the weight of you weapons and armor is easy to go past that threshold.
Edit: Also, how do I get back to the shrine when I travel places? In BB I'd clear an area to get souls, and then run back to the lantern to level up and buy stuff, and just keep repeating. Do you have to find the new bonfire every time you travel to a new place?
Every new place always has a bonfire at the start, don't worry. You can always use a Homeward Bone, which can teleport you to either the last bonfire or the firelink shrine.