So as you can see I want to be a mage/thief/ and assassin character but should I be a Dark Elf, or High elf for this?

Dark Elf.

I'd agree Dark Elf. Unless you want to really concentrate more on mage than thief. Then, I'd go high Elfr because you start with a really nice +50 Magicka racial bonus.

Dark Elf, Khajiit or Argonian.

High Elf is best for pure mages/spellswords because of their unique attribute bonus.

Yeah Dark elf gets a nice median of magic skills and stealth skills from the start.

Race doesn't matter. Skill bonuses are negligible. But if you already asked...

Dunmer are naturally skilled in both stealth and magic and fit into the role of a thief/mage. Their fire resistance will help when fighting fire dragons. Their racial power seems to make more sense for a thief than Altmer's.
Altmer have a nice +50 magicka bonus which will we useful if you're planning to rely on Illusion. It will free some points to spend on health or stamina, too.

Looks like Dunmer is an overall better choice.

Keep in mind that you can overcome any racial traits/starting skill boosts.

If you're not just asking about race to be "in character", then go with a race that appeals to you the most.

When i make my chars i always check up on this site for stats/start bonuses
It's just to check what race got the must plus on those stats you want and what race spell is must usefull.
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http://elderscrolls.wikia.com/wiki/Classes_%28Skyrim%29

Argonian = (using Oblivion stats as guide) excellent assassin but also not bad bonuses with magic and potions. Females better with magic than male. They lack in destruction but what assassin / thief would use destruction? You don't get +6 damage for throwing a non discrete fire ball.
Argonians get magic bonuses on top of stealth assassin. I like to play Argonians as sneak assassin / thief with illusion and potions.

Altmer are obvious best with magic. but aren't bad with stealth (average). Obviously good with invisible magic etc and fortify potions

Dunmer has destruction magic bonus but why use destruction when they are good with both sword & bow. I prefer to use Dunmer with sneak, sword, bow & conjuration type play.

this. argonian is right for everything.

Heck, you could make a pretty nice mage/thief/assassin out of a Nord if you want...so many cold attacks in the game, that 50% cold resist comes in handy on any build.

argonian is the way

Argonians are alright at magic AND can breathe underwater. Not too bad. Not exactly great though either.

Altmer are good for magic, but sneaking isn't really their thing. On the upside though, they get a scaling bonus due to their size which lets them move faster AND hit harder (yes, really).

Dunmer are resistant to fire, specialise in Destruction magic, and are good with blades. Probably the best all-round choice.

Khajiit are the best rogues / assassins in general... not quite so hot on the magic side of things. Being able to see in the dark is MASSIVELY helpful though.

And then there are Bosmer... though be warned that they're slower and don't hit as hard as other races for those bizarro size-related reasons. They are pretty much the best choice for RANGED assassins though.

Breton... the magic resist racial is the best in the game early and late. IMO the best for any class. This frees up an enchanting spot late game.

dark elf. This is almost stereotypical dark elf build

Dunmer is by far the most rounded of the races.

Dark Elf. Thief/Assassin totally suited for them. Thief/Assassin/Mage suits them as well, but you will find that you use less spells than you might think, unless you are going more for a Mage aspect. Usually will use more Bows, daggers and swords... compensate with some Conjuring spells for assistance, Detect Life to see through walls and plan tactics, Dragon SHouts for defense and added abilities, and Alchemy for the poisons and potions. Just fits the Dunmer perfectly.

If going for more of a Mage/Assassin/Thief, Dark Elf are again perfect... but can use High Elf and Breton to some great advantage as well for their Mage aspects.
But truly, having made a Thief/Assassin... Dark Elf... and loving the character/race... I really do use little to no magic other than the ones I listed above.

This
Very useful ability for any character type

Well I guess I'll stop on my 71 Nord and make a Pure High Elf mage and a Dark Elf.