A 3rd degree Gardnerian Witch would have to start out from the very beginning in the Feri tradition. Credits don't tend to transfer, although there are sometimes exceptions to this. Each tradition has its own training, teaching, mythos, and mystery. Group ritual is a totally different experience from solitary ritual, and the fellowship, feedback, and encouragement of group training can be very rewarding.
Once Pagans had to seek initiation from whatever group was nearby and known. This is slowly changing and the seeker has the opportunity to "date" different groups until they find one that fits them. While the private initiation of a God/dess can come unbidden, the initiation into a group is something in which the initiate has modicum of choice. While the group must accept you for that initiation to happen, you always have the opportunity to decline.
Pagan initiation is often secretive and this is because in such a close-knit spiritual setting trust is incredibly important. If you don't feel comfortable submitting to initiation, whether it be to swear an oath under the sky or to enter a circle naked and blindfolded, then you don't have to do it. The rise of Modern Paganism has given us diverse and vibrant religions to choose from. There is something out there that will suit you if being in a religious group is important to you.
I have spent six months training in a tradition before I finally admitted that it was wrong for me, however badly I wanted it to be right. Rather than push through to initiation with a heart full of reservations I walked away, and surprisingly found myself in a tradition that I'd never previously considered, yet felt at home in.
I think those of us who receive personal, private initiations have somewhat of an advantage. Having already faced our dark night of the soul, which for some of us may be like a shamanic rebirth, we are both more comfortable and more wary of future initiations. Which is a good thing.
It's very tempting to read a book on Wicca and rush out to petition for initiation to the nearest coven, but sometimes you need to do some inner work first before you jump into coven work. Initiation of any sort is a calling and that's not to be taken lightly. You may spend years seeking your calling only to find it was not what you expected when it comes to you. You may long for the knowledge of a tradition yet find the available teachers of that tradition to be people you don't actually want to be associated with.
I've spent ten years in private practice and study and still haven't been initiated into a tradition. Maybe sometime next year I might petition for initiation into the tradition I'm studying but for now there is no hurry. What I have known of private initiations gives me no desire to rush into a group initiation. To bind yourself to something is a strange, rewarding, and difficult thing. When you have the opportunity to make the choice to serve, make it well, with surety and confidence. Be sure you can handle the responsibility initiation grants, and that you are ready to venture into the unknown. Besides, initiation may find you all on its own while you're looking for something else.
The Witch Cult in Western Europe by Margaret Alice Murray
Initiation: Pausing at the Threshold by Sunfell