If used separately (no fortify crafting skill) armor/potions, on a thief/assassin, will they be really that op?

Alchemy; useful potions (resist mag and fire/frost/shock vs mages/dragons), healing, paralysis
Smithing, powerful legendary smithed daedric/dragonbone weapons
Enchanting; pretty much frost/fire enchanted bow

Im also playing on legendary

For legendary, it depends on how much you stack.

My current character only uses Smithing and Alchemy. Smithing because warriors are the best, and Alchemy because I plan to make him a very strong Dragonborn. That, and his Thu'um is his only form of ranged attack.

On legendary dont go over 500 damage.

I play Legendary as well, my enchanted bow does around 800 damage and my other stuff is like 500-600. Armor rating is like 2700 which I think is unnecessary because there is a cap at like 600 I believe. If you don't want to be OP then go a little less then I have because I am fairly overpowered. Nothing can really kill me to be honest. That might also be because my character is level 252 so I got around 1800 health. Leveling this high makes some things very powerful in terms of health though. The Ebony Warrior and some random wanderers I find throughout Skyrim have bogus amounts of health, and at that point even though my bow does 800 damage, the damage done is quite trivial. If you don't want to be overpowered, I'd say go a couple hundred lower than what I have. Maybe leave out potions when smithing and just wear enchanted armor with fortify smithing.

I just use smithing and enchanting on my 2h warrior and it works pretty well on legendary. I don't use any alchemy or drink any potions. It's a decent compromise and you can get strong weapons without zomg 1000 damage craziness

Well, if you don't perk it, you can't make those really powerful potions and enchantments and weapons. (I usually perk one skill, and use the other two as back-ups with only a perk or two in them. I don't play on Legendary though, I shift between Expert and Master, you can probably perk them a little more on Legendary and still not become OP)

On legendary probably not, Master is debatable. Any other mode would be OP. I would avoid Chaos Damage, that will probably be OP.

Personally what I would do in that situation is get the skills up to the required values, make a save and then create some weapons and do some test runs. If you see your self clearing places too easy and it's not because of player skill at the game or other mechanics (Like jumping up to a high place and pecking enemies with Fireball/Arrows) then you may want to go lower on the damage. Reload the save and then try again.

As long as you don't use falmer craft gear or the loop exploit, the game is pretty well balanced if you stick with legendary. Although an assassin is inherently powerful, so I'd just mind the dark brotherhood gloves.

And using just ONE crafting skill on Master...? Thinking of Enchanting for Spellblade champ, mostly.

The real imbalance issue comes into play when one continues to improve their gear past Legendary. Armor is capped, Magic is capped and weapons should have been capped as well. Using all three crafts doesn't necessarily demand that one is,or will, break the game.

Then there is the use of glitches, like the Restoration loop. People use that junk and then throw a temper tantrum because they completely removed the challenge from the game.

Now as far as the Assassin, one shots. Well, that's kind of the idea. The sheer fact that the PC gets in close enough to cut the throat should warrant the ability to one-shot the enemy, in the first place.

It also comes down to you particular character and IF the use of a skill is fitting to his or her personality.

Here's a simple easy way to avoid becoming OP'd if you use all three crafting skills.

Are you ready? It's super secret. I'm not sure you'll be able to handle it.

Are you ABSOLUTELY certain you can handle it?

Okay... ready?

REALLY ready...?

Mind-blowing, isn't it?

Grinding isn't an issue either. I mean, you are leveling up your character anyway. If you take one skill directly to 100 you will be around level 17. Two skills and you are level 21. Three skills to 100 is still level 25.

Even then, you can be in trouble because your primary damage skill is severely lacking. Say you get it to 100 as well. Ok, yare ahead of the leveling curve for enemies. You are still going to keep leveling, somewhat. Lockpicking and Speech is not controllable. Unless you specialize your character with three crafting skills and a weapon skill thus capping them by level 28, you are going to at least get into the low to mid 30's.

That's some seriously restricted roleplay.