Get an Amulet of Health
By which I mean start with one. It sets your Con to 19. You need one. Then you will be a god amongst men. Since you're playing Tomb of Annihilation you may want to know:
There's an amulet of health located in zone 22 in Papazotl's Tomb. It's the item that gets you possessed by Papazotl. That's fairly late in the game, but it might make it easier to talk your DM into letting you start with one.
Assuming you can't get said amulet, however:
An Abjurer can get some staying power without constitution
An abjuration Wizard gains the Arcane Ward feature at 2nd level, which provides twice your level plus your intelligence modifier in extra hit points that replenish as you cast abjuration spells. At 6th level with your base stats this is at least 16 extra maximum hit points. That's a massive improvement over the measly 8ish max hp your wizard would otherwise probably have!
Being a wizard also lets you ameliorate the effects of your low constitution a bit: while your d6 hit die will almost never outweigh your negative Constitution modifier, your minimum HP gain per level can't go below 1. This is particularly good since you are, for some reason, focused on median rather than mean hit points-- while your mean or expected hp at level 6 from wizard levels with 3 Con is 7.5, your median hp is a whopping 2 points higher: 9.5. This is because there are a very limited selection of values your hp can be, and there's about an 80% chance it's one of the bottom two.
A Warlock can charge Arcane Wards all day, every day
Warlocks get the only unlimited at-will spellcasting ability we can use to fuel Arcane Ward: the Armor of Shadows eldritch invocation. This lets us "cast an abjuration spell of first level or higher" an unlimited number of times without resting, plus we get to up our AC a bit. It's only 2 hp per casting, so it'll take 48 seconds to fully recharge a downed ward, but if you need your ward back up faster than that you're better off casting one of them fancy higher level spells anyways.
It's two levels to get invocations, so you'll be War2/Wiz4. Eventually, in the far future, you'll really want to replace those levels in Warlock, but that's at level 18, which is far enough out I think it's a worthwhile trade-off.
Your levels in Warlock also give you a patron, which should be either the Raven Queen or the Undying.
The first gives you access to the Sanctuary spell, which is a nice effect for a support caster, and also is an abjuration so it'll recharge your ward a tiny bit when used, as well as a cool raven familiar that stacks with other familiars you might have except that it's totally not a problem if it dies repeatedly. The raven is particularly good since it boosts your perception score (by 2 in your case) so you are less likely to get one-shotted by an unexpected ambush.
The Undying instead gives you a sanctuary-spell-like effect that's always on, but only functions against undead and only until they first make the save. Tomb of Annihilation has a lot of undead baddies, so this is pretty comparable, but that's all it's going to give you. Your call on whether saving the allowed-to-cast-spells-this-round flag for something else is worth it.
Note that Warlocks have a d8 HD so you suffer more from your negative Con score: a regular character would gain around 1 more hp per level from that change but you only gain around .58 hp per level more over those two levels. Our expected hp and median are still a bit higher this way, of course, and we end up with an expected hp of 10.42 and a median hp of 12.5.
Our second invocation can be utility stuff like Eyes of the Rune-Keeper or Eldritch Sight, or 8 more hit points via false life via Fiendish Vigor.
Half-orcs get a do-over
Half-orcs (and Minotaurs from Planescape: Amonkhet, but those swap out darkvision for a garbage natural weapon) get +1 constitution and the ability to instead be fine the first time they would be reduced to 0 hp or below each long rest. This doesn't trigger if you are insta-killed by massive damage, which is a significant concern for you since that's, like, 45 damage and there are CR 6 creatures that can actually do that in a single round if you let them (don't get charge-stomped by a mammoth).
The final recommended build would look something like this:
temp hp: 8
arcane ward: 17
Background: Stojanow Prisoner
Skills: Perception, Deception, Investigation, Arcana, Intimidation
Tools: Thieves' Tools, Gaming Set
Patron: Raven Queen
Invocations: Armor of Shadows, Fiendish Vigor
Eldritch Blast, Minor Illusion
Pact Magic Spells Known:
Sanctuary, Armor of Agathys, Hellish Rebuke
Control Flame, Encode Thoughts, Shape Water, Blade Ward
Wizard Spells Known:
Comprehend Languages, Identify, Fog Cloud, Tenser's Floating Disk, Find Familiar, Grease, Unseen Servant, Lonstrider (+ whatever you can get from scrolls, NPCs, etc)
Flock of Familiars, Detect Thoughts, Rope Trick, Enlarge/Reduce (+ etc)
Out of combat, you are fairly tanky since your hp completely replenish after each trap, fall, firewalking exercise, or what have you.
In combat, you want to be in the back and avoid damage as much as possible, using an unseen servant and/or flock of familiars and/or your sentinel raven to control enemy movement options. You can cast enlarge or false life on an ally to buff them without losing sanctuary, and you can cast sanctuary twice per short rest to discourage enemies from attacking you. Armor of Agathys can single-handedly eliminate hordes of weaker enemies who attack you, because your Arcane Ward will ensure that the temp hp from that spell stay around until you're basically dead anyways. Eldritch Blast provides an option to deal okayish ranged damage if it comes up, and something to do while sanctuarying when defence isn't urgent enough to warrant a blade ward.
Out of combat, your wide variety of utility spells, massive perception abilities, and proficiency in Thieves' Tools will let you act a bit like a rogue.