Fallout 76

Legendary cores are a new component in the game that allow you to modify weapons and armor into legendary versions of that weapon and armor. You can create a new piece of weapon/armor (including power armor) or use an existing piece of weapon/armor and then go to a workbench, switch to modify, and then mod that piece into a one star, two star, or three star legendary piece (you need to make ONE OF EACH type of mod [1*, 1*2*, and 1*2*3*] to get the achievement). Alternatively you can mod an existing legendary piece into a new one with one, two or three stars. Note, however, that any modification of existing legendary pieces will replace your current modifiers! So don't go doing this on a bloodied explosive less vats fixer for example. It will not keep any of your current modifiers!

To add/change legendary mods for weapons/armor you will need the following amount of legendary cores PLUS legendary modules from the Purveyor:

Weapons / Power Armor
1* = one legendary core, two modules
2* = three legendary cores, three modules
3* = five legendary cores, four modules

Armor
1* = one legendary core, one module
2* = three legendary cores, two modules
3* = five legendary cores, three modules

Legendary modules cost 50 scrip each from the Purveyor (at the Rusty Pick in the Ash Heap) and you are limited to purchasing 10, HOWEVER, if you want more, just go to another server :). So in total you need 9 cores and 6-9 legendary modules (costing 300-450 scrip) to get this achievement. Also you can modify the same weapon/armor over and over so you don't need to have three different pieces.

Event Drops

The following events are guaranteed to drop a certain number of cores:

-A Colossal Problem - 8 cores
-Encryptid - 8 cores
-Free Range with 3 Brahmin left alive - 8 cores
-Project Paradise - with 3 creatures alive - 8 cores
-Scorched Earth - 5 cores
-Campfire Tales - 2-6 cores

The following events give you 1 core (48% chance), 2 cores (32% chance), 3 cores (20% chance):

-Decryption (only if you finish with Elder Rewards under 8 minutes!)
-Distinguished Guests
-Feed the People
-Guided Meditation
-Heart of the Swamp
-Jail Break
-Line in the Sand
-Lode Baring
-One Violent Night
-The Path to Enlightenment
-Radiation Rumble
-Swarm of Suitors
-Tea Time
-Test Your Metal
-Uplink (only if you finish with Elder Rewards under 8 minutes)
-Uranium Fever

Others to be updated later

-Fasnacht Day
-Grahm's Meat Cook

Your best bets are, in order:
1. Decryption / Uplink under 8 minutes (Daily Op)
2. Free Range
3. Scorched Earth
4. A Colossal Problem
5. Encryptid
6. Campfire Tales
7. Everything Else

once you've acquired legendary cores from daily ops or scoreboard (comment to tell other drop locations) you'll need 10 legendary cores for this.

go to a crafting table and choose modify. select which weapon / armor / power armor you want to use and go into editing it. there will be an option at the top of the new menu, then select whether to choose a random 1 star, 2 star or 3 star (1 core, 3 cores or 6 cores) to get this achievement quick select on any gear choose a random 3 star then on same item choose a random 2 star and again choose a 1 star and that is it done.

Here is my 100% Achievement plan for Fallout 76. If you need help on an achievement, hopefully it will help. And let me know if you need any help. THIS achievement is at 30:16 in the video.