If I found a doctor who claimed he could make it grow, I'd start running as fast as I could in the opposite direction, after checking to be sure my wallet was still with me.
We all grow at different rates and comparing yourself to a buddy is probably the worst thing you can do for your own peace of mind. You have several years of growing left.
I know you posted some time ago, I hope you have found some resolution to your concerns, but ...
First ... The length and girth (thickness) of your penis is not an issue unless you believe it is. At your age, it is possible that your penis will continue to grow, especially as you begin to use it for sexual activity. Penises, just like other parts of you, come in as many styles, shapes and sizes as people themselves! I've seen smaller than yours and bigger. You may see other boys who seem to be larger, but everyone is somewhere along the "size" spectrum, and just about all of them function just fine.
So, size is not the point, necessarily, unless you are trying to have intercourse with someone who is very heavy.; in other words, they have a lot to get past in order to push your penis inside. That situation is not very likely, so let's deal with more "routine" sexual matters here. First of all, is it FUNCTIONAL, right? If you can pee through it, if you can masturbate to orgasm and ejaculate (when your body has begun doing that) , if it feels good when you play with it, if it provides you with wonderful sensations, then no worries, you -- and it -- are doing just fine. As for ejaculation and pre-cum, everyone's body matures differently from everyone else's. No pun intended, but if you keep at it, it will come in time -- it will happen. If you are truly concerned about it, you should ask your doctor. Don't be embarrassed -- just think of it as a piece of equipment you own, and the doctor is the mechanic!
THE ABILITY TO GIVE PLEASURE TO OTHERS -- If you are heterosexual, remember that a girl's primary organ for sexual pleasure, the clitoris (which is located at the top of the vulva, nestled between their vaginal lips) is OUTSIDE her body, meaning that your erect penis is certainly capable of pleasuring that part of her (along with your gentle fingers and your tongue, by the way). Rubbing your erect penis along, around or against her clitoris can bring both of you well along to orgasm. Next, her G-SPOT (another major organ for her orgasm) is located just inside the top of her vaginal opening, kind of “behind” her clitoris.( If you put your finger in there it feels rough, like the roof of your mouth.) Your 3" erect penis is certainly capable of reaching it, as "Dalphy" has pointed out here. During intercourse, try thrusting upward rather than inward. This will force the head of your penis against it and should excite her nicely. And you don't need your penis, necessarily, to stimulate her elsewhere on her body: kissing her mouth, her breasts, nipples, belly button, inside her thighs, nuzzling under her arms, licking her ears, her toes, etc etc etc. – pretty much anywhere!
A lot of that also holds if you are homosexual, by the way. Male bodies have similar sensual places that you can stimulate. The same no-penile methods of stimulation are crucial to a complete lovemaking / sexual experience, regardless of gender. And of course, much of homosexual sex centers around oral / genital contact, and very few guys will turn down the chance to give oral sex to someone because they are too small. Too large, maybe, but never too small. So, no worries there.
But most important of all, as you explore your sexuality, is to remember that the object of your desire is another human being, with their own fears and doubts, and their own expectations, feelings and desires. Sex is a dance you do, even if it's with yourself right now. It's not just the technical bits about what part goes where. The size of your penis is WAY less important than the kind of person it's attached to. Are you kind? Are you gentle? Are you considerate and thoughtful? Are you honestly curious about THEM? Do you try to anticipate the other person's needs and satisfy them? Do you LISTEN and observe what they are telling you, and then do you adjust your behavior to match them? Is THEIR pleasure more important than your own? You will find that making someone feel loved and appreciated -- especially during sex -- will bring YOU great pleasure as well... well beyond just the chance to shoot some sperm out of your penis.
Hang in there, I think you (and your penis) will do just fine!