As someone who primarily writes series that are anywhere from 3 to 7 books (so far), I would recommend writing one at a time. Stick with the current one for now, because you'll really be able to flesh out the story and world-building, rather than trying to write multiple books at once and risk losing continuity with the long-term story. If you try to work on more than one at a time, you might find yourself flip-flopping back and forth trying to keep all the histories, events, and arcs aligned as your multiple stories evolve. Sticking to one at a time gives you a more decisive timeline or plotline to follow with prequels and sequels.
You should ask yourself, why is it you think you need a prequel or sequel? If you are just trying to flesh out the history of events, that can be woven into your main story/series. If you have events or characters that you want to give more attention to or that more detail is vital to the overall story, then that could be grounds for a prequel or sequel.
I have to say, I never do enter into a book thinking it is going to be a series, but they just tend to evolve that way. Often, I'll have a core trilogy or something, and then I will write additional series in that same timeline/universe but that follows a different set of characters at a different time (whether it be past or future). Usually, these "spinoffs" are inspired by historical events or characters that I mention or reference in the main series and want to give the proper attention to in a separate book, or they are the continuation of a character who was peripheral in the main series and whose story diverges from the main series.
If you aren't familiar with it, check out Anne McCaffery's Dragonriders of Pern series. She started with three books in a trilogy with that series, and then expanded it to writing one offs and other trilogies that spanned a time frame of several thousand years. Even her son and daughter helped her continue on the series in her later years and since her passing. Some of her one offs overlap with other time periods, and some of her trilogies coincide with the original trilogy with a different character focus, others delve deep into the history of her world, and some will follow a specific character that was a peripheral character in another book. Her prequels and sequels cover a large span of time before and after the original trilogy.
That might sound overwhelming, so I hope you understood the overall point of that. Basically, she started with one trilogy and just kept expanding on it in whatever direction made sense.