You can join the challenge at any point. It doesn't really matter at what point you start, although your very first post probably shouldn't be about joining the challenge. Since you already have half a dozen posts, that shouldn't be a problem for you.
Writing 3X per week is fine; there are several of us that do so. You will grow faster and larger if you write every day, but it is a matter of how much effort you are willing to make, how much time you have to devote to it, and what your ultimate goals are. I've accepted slower growth for more time to spend on other projects and hobbies.
Besides the work you do on your own site, there is a pretty proven path of success. It involves being active on these forums and commenting on other blogs. The next time Yakezie teams get organized (keep an eye in the Blogger's Lair sub-forum in a couple months), join. Teams are basically mini-Yakezies of mutual support including socially sharing and commenting on each other's posts. It's a good idea to participate in carnivals, both submitting to and occasionally hosting. Both ways bring backlinks to your site.
Also, it's a good idea to make a signature for the forums here and put your URL in there, so it shows up on every post you make. The forums are well indexed by Google and, even though the links are nofollow, if somebody searches for something and comes to a forum post you commented in, that's a free eyeball to your link.