I used henna on my dark brown hair a couple years ago. Quality henna will make your hair stronger and thicker. I just wrote a hubpage article on using henna to dye your hair:
Chemical Free Hair Color: Steps for Using Henna to Dye Your Hair
I also have 3b hair. Hope this helps!
3b curls
Products I'm using:
Shampoo 1-2x/week: Organix Coconut
Co-wash: Suave Naturals Coconut
Conditioner: GVP Conditioning Balm
Leave-in: Giovanni Direct Leave-In
Gel: LA Looks Sport

How to HENNA your hair: http://annieshealthtalk.hubpages.com...-Dye-Your-Hair
"Sh*t Curly Girls Say": http://www.youtube.com/watch?v=emqRg3t3mng