Re: Ghostbusters 3
In fact, the Gundam franchise is difficult to classify as actually having any true reboot. In airing order, you have the original Gundam series (a.k.a. Gundam 0079), followed by Gundam Z, then Gundam ZZ, then Char's Counterattack, 0080, 0083, F91, and Victory Gundam, all existing within a single continuity. It wasn't until G Gundam that you could argue the series got anything close to a reboot, although the naming and dating conventions for these series make it abundantly clear that we're not actually "rebooting" the Universal Century continuity, but rather are talking about some alternate timeline/universe.
While it's true that many of the series, OVAs, and films draw inspiration (sometimes only mild, other times very heavy) from the original 0079 series, I wouldn't call them reboots, at least not within the Universal Century continuity. Moreover, the UC hasn't been rebooted yet, in the sense of implying "We'll never be returning to this universe again." Gundam Unicorn just wrapped up, I think, last year (or two years ago? I dunno -- I haven't seen that one), and it was told within the UC continuity. So, the UC still "exists" and may even be revisited again in the future, even while these alternate universes are explored.
The Gundam meta-series is kind of a weird animal to classify as far as reboots go. I'd say that subsequent non-UC series are more often better categorized as "reimaginings" of the original stuff. For example, Gundam Seed is almost identical to 0079's plotline, but it changes certain aspects as well. Gundam Seed Destiny bears some resemblance to Gundam Z, but diverges more than Seed did from 0079.
Within the UC, each independent series had certain familiar touchstones (e.g. young newtype protagonist, masked villain pilot with mysterious past, etc.), but beyond those broad strokes, the series are often very different in story and tone, and from 0079 up to Victory Gundam (and later Gundam Unicorn), they're all meant to be told within a single continuity. I wouldn't consider any of them reboots or reimaginings, so much as they are simply stories with similar elements running through them. I mean, geez, you can hardly call Victory Gundam a reboot of Gundam ZZ, ya know?