There are many tools and tricks authors use to nail down a fully-realized character. Some write detailed character profiles, some keep a list or spreadsheet of all the character details, and some use other inspirations. For me, I have a sparkly purple notebook that holds my entire writing world. Every character from every story is listed in there with eye color, hair color and distinct traits like biting her bottom lip wihen nervous or leaving the room instead of facing conflict.
In addition to that, I get a lot of inspiration from music. As I listen to songs, i imagine the characters in my current works and if the words or mood of the song fit them. I usually find one or two songs that I mentally attach to that character, and whenever I'm ready to write, listening to the song reminds me of the uniqueness of their personalities and situations. For example, with my upcoming story Mistletoe Madness, Jordan Sparks' No Air and Next to You make me think of Sage and Scarlett, and 5SOS's If Walls Could Talk is my theme song for a character in my fantasy book Son of Water (coming in 2019).
I share all of this to say, there is no right or wrong way to develop your characters. Do whatever it takes to make them real to you. If you feel their "realness" then your readers will, too.
Happy writing, and go create those amazing 3D characters!
Feel free to share in the comments if you have any tips or tricks you use.