I am also a 3a with fine and thin hair. My hair has always been thin (as in, not much of it), even when I was young. My mom has the same hairtype, so I am convinced that it's genetic.
If you are trying to grow your hair out, I have found that avoiding heat styling and taking vitamins is the best route. It can be frustrating because you don't notice results on a day to day basis. But one day you will be comparing your hair to an old photo (or something like that, lol), and you will see that it's grown a lot! At least that is how it works for me. I don't have a magical growth solution to tell you about, but babying your hair, getting proper nutrition, and taking care of your scalp will help.
For products, you want to avoid breakage as much as possible. With our hair type, I have found that I am better off using some styling products with silicones. After not using any silicones and doing the full CG routine for more than a year, my hair was more damaged than ever. It's because my hair is SO fine and thin that everyday things would damage it. I need the silicones to protect the hair. The damage caused by washing out silicones was less than the damage I had from every day stuff when I didn't use silicones. I use and love the Kinky Curly leave-in you mentioned, and I also like Dermorganic argan oil treatment (a serum-like product). My favorite product for hold is PM The Cream (I actually use the generic one from Sally Beauty).
3A curls, fine texture, thin (it's always been that way). A few inches past shoulder length and would like it to grow very long.
Sulfate-free shampoo: Jessicurl Gentle Lather Shampoo, Elucence, Bedhead Superstar
Conditioner: Jessicurl Too Shea conditioner, DermOrganic Masque, Redken All Soft
Style: DermOrganic Argan Oil Treatment/Serum, Kinky Curly Knot Today leave-in, Kerastase Nectar Thermique, Oyin Hair Dew