Is it bad to get your bachelor's, master's, and PhD at the same school?

If it's ranking in the top 3, it would definitely make more sense to do your PhD there too rather than at a low-ranking university. However, if you do well on your Masters you could consider studying at another top 10 uni for your PhD - you would then increase your network size and have more contacts in academia - an advantage if you plan on continuing as a third-level educator/researcher.

Nope. If you're in a top program, how would it look better to leave that school and do the rest of your training at a school with a worse reputation? It's very common for people to complete all of their training at the same school, especially if it's a reputable program (eg are you going to knock a doctor who went to Harvard for both their BA and MD?).

When it comes to applying to PhD programs, you should definitely apply to several schools (including those ranked higher than you) to make sure that this school is still the best personal fit....when you get to grad level training, you'll find that schools each have their own niche in which they excel (based on what faculty is where).

It will depend on the subject, but typically getting your advanced degrees at the same school as undergrad is discouraged. A BS/MS program would be exempt from that, but if you're going on for the PhD, your professors will encourage you to go elsewhere. If it's at the top, it won't look so bad if you stick around - but if it were not top-ranked, it would look a lot worse.