Background is being worked on, nothing set yet.
This is probably a common multiclass, but I'd like some input on the mechanical side (and not the RP side at the moment). This is also assuming Invocation prereqs are Character Level based, not Class Level based.
Class Breakdown;
Paladin: 2 (Divine Sense, Lay on Hands, Fighting Style, Divine Smite 1st level spells)
Great Weapon Fighting
Warlock: 4 (3 Cantrips, 5 Spells, 2 x 2nd level slots, 2 Invocations, 1 Ability/Feat)
Patron: The Fiend
Pact of the Blade, Thirsting Blade, Lifedrinker
Sorcerer: 14 (14 Points, 2 x Metamagic, 7th level spells, 3 Ability/Feats)
Origin: Draconic Bloodline (would love more options, would vastly prefer an Infernal/Celestial bloodline)
Multiclassed Spell slots: As 15th level caster (8th level slot)
Feats;
Elemental Adept
Great Weapon Master
(Maybe Ritual Caster)
Paladin to start with, then three warlock levels to get Thirsting Blade at 5th. The last Warlock level will be taken at 12th level for Lifedrinker.
For Origin, I'm leaning towards Fire, to synergise with Hellish Rebuke (and overall theme). But would like combat actions to be Action: Attack (2 Attacks), Bonus: Quickened Spell/Cantrip. So am after a Cantrip I can synergise with Elemental Adept and Draconic Origin. Firebolt would be the choice, but as a Ranged attack I'd be at a disadvantage using it in Melee.
I suppose I could alwaysmove away and provoke an AoO to use Firebolt without Disadvantge. Would let me trigger Hellish Rebuke.
Fire would also synergise with spells like Sorching Ray, etc. If only Sacred Flame did a mixture of Radient and Fire damage. 
How does Dark One's Blessing work? How long do the Temporary Hit Points last?
Thinking about Ritual Caster, just incase there's any useful Rituals of spell level 8 and 9, as the feat would allow me to use these.
Edit: I should add that Crossbow Expert has been ruled to not help spells ignore the disadvantage of using a ranged attack in melee. Maybe I should try to get Spell Sniper to have that benefit.
I was thinking of a similar build. If you go paladin, you should at least go to 6. Channel divinity is good, the charisma bonus to saves is the best feature of paladins also you get 2 attacks from paladin. That means you can go for the book warlock and pick up shilliai(spelling?). Then use charisma for hit and damage. You should also consider bard (lore) build, the inspirations are amazing.
My build would look something like this
Paladin 6, warlock 4, sorcerer 10 Sorcerer is great for the spell points and quicken, but it does not get much better after 6. You would only need charisma for hit, damage, saves and spells.
And have to use a staff. So no GWF/GWM, and Lifedrinker.
I'm not sure picking up shillelagh for Charisma to hit is worth it over going Devotion/Vengeance for Cha to hit / Advantage.
Sorcerer 14 also squeezes in Wings right at the end.
Could the change the focus to be more martial, and go Paladin 11 (for Improved Divine Smite), Warlock 3 (with third level at 12 still for Lifedrinker) and Sorcerer 6. This still nets Elemental Affinity, but reduces spells known to 3rd level and works as a level 11 Multiclass Caster (1 6th level slot).
Didnt the developers confirm that Invocations are gained by Warlock level, not character level? Its kind of stupid if you can take the best of the invocations by just going a few levels into warlock.
They confirmed that they weren't going to confirm anything, which is really helpful (especially for organised play and the likes).
Temporary Hit Points last until they're lost.
Awesome. Dark One's Blessing seems quite nice. Might be worth looking at Necromancer for Grim Harvest to pair with it.
Anyone got any thoughts on the two versions?
Paladin 2, Warlock 4, Sorcerer 14
or
Paladin 11, Warlock 3, Sorcerer 6
Looks nice. I've been tossing around some ideas for what to do for a melee warlock. I'm not really sure what I want to add. I'd like to go for as much THP/life drain as possible. I do wish I could use grim harvest on melee attacks. I tried to start a thread on class ability blending feats, but no one was interested. Still, I think it'd be a fun character.
Yeah, I'd still want Sorcerer 3 for Quicken.
MCing with Sorcerer and Wizard (Necromancer) would give me access to only low level spells, but still enough spell slots to fuel Divine Smite / Quicken.
I would still have access to Haste, through Paladin (if I went Vengeance).
So maybe;
Paladin (Vengeance) 11, Warlock (Fiend/Blade) 3, Sorcerer (Dragon) 4, Wizard (Necromancer) 2
Would still be an 11th level Multiclassed Caster, and get one 6th level slot.
Edit: I would also gain an Invocation, as all I need is Lifedrinker, and wouldn't require Thirsty Blade. I'd lose Elemental Attunement though.
While I'd lose the extra fire damage to Hellish Rebuke, I coudl lose Elemental Attunement and use the free Invocation to pick up Agonising Blast. Could have Shocking Grasp and Eldritch Blast as my two go to Cantrips.
A Question about Ritual Caster and Wizards. I was looking at the Ritual Caster feat to allow me to cast 9th level Ritual Spells.
But looking at the Wizard class, it appears you can cast any Ritual Spell you have in your spellbook, regardless of level, but you could only copy spells into your spellbook of a level you have slots for.
It this covered by the Multiclass Preperation rules? So even though I have a 6th level slot, I would use my Wizard Slots to see which spells I could copy into my Book?
If so, the Ritual Caster feat might still be attractive.