Hello, Everyone

I'm a player in a party of 6 people currently about 5-6 sessions into a campaign. We're currently lvl3. Our party consists of a Paladin (Oath of ancients, halberd/polearm master), Cleric (Tempest), Wizard (Transmutation), Monk (Kensei), Fighter (arcane archer), and myself, Sorcerer(Shadow)/Warlock(Hexblade)/Bard. The 'frontline' usually consists of me and the paladin, and sometimes the monk.

We are playing with a custom rule for flanking, basically as it works in 3.5. With the rules for opportunity attacks as they are in 5e, it's often very easy for enemies to move into a position to flank. It's not uncommon for fights to be initiated by an ambush on our enemies' part. This makes it difficult to stay in a tight formation to help avoid flanking. We were also allowed to take a feat at first level (to help better flesh out our characters mechanically in the beginning). I took war caster.

I currently have 28hp max (18con, max hp from sorcerer at 1, average hp for the other two levels). I don't feel that that's bad for 3rd level. My AC is only 14(shield 15dex), which ins't great, but I'm planning on taking the mage armor invocation next level. I keep the shield spell ready for nasty attacks, and I also have the strength of the grave feature from shadow sorcerer to try to stay up after an attack that would down me.

The party has decent access to healing, between the paladin, cleric, bard and the monk (aasimar).

The problem is that usually 1 or 2 of us will go down in any given encounter. I would say, at least half of the time, it's from a critical hit on an enemy's part. Advantage from flanking is probably a large factor in this. Last session I got hit for something like 37 damage in a single hit from a skeletal minotaur, without being flanked. Neither shield nor strength of the grave offers any protection from critical hits. Even if I had taken something like Tough for my feat, it would still have downed me.

What are some ways I, or others in my party, could help to avoid enemies hitting us with critical hits? The first things that springs to mind would be the halfing racial feat to force enemies to reroll, but none of us are halflings. The other would be to try to impose disadvantage on enemies' attacks, and obviously to avoid getting flanked. What are good ways for us to give disadv to enemies? Magical darkness isn't something I'd like to rely on, as at least a decent chunk of the enemies we fight have some form of blindsense.

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I appreciate any help

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EDIT (add some more info): We're doing an aquatic campaign, and the DM has a rulesbook for it. One of the rules is that wearing armor while swimming gives huge penalties (The paladin and cleric can breathe water, so they don't risk drowning). I'm probably going to try to get a breastplate when I so it's an option, but we haven't been able to afford it yet.

I didn't know about adamantine armor, but I'd guess it'd be pretty hard for us to get our hands on, assuming we could afford it.

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On a related note. I don't feel like I know when to use dodge vs blade ward. I guess it theoretically comes down to their damage output compared to their chance to hit.