Of those three, if I had to narrow it down to one, I would say Yamaha, aAlthough a high-end flute from one brand might be quite a bit better than a beginner flute of another brand. It is really difficult to just say one brand is "better" than another.
For example, Jupiter, which is by all condsiderations a lower-priced (some say "bargain") brand, has a few higher-end flute lines, called DiMedici and Capital, which might be considerabely better than a low end Yamaha.
Armstrong should only be considered a beginner or perhaps intermediate brand.
Yamaha has flutes of beginner, intermediate and professional quality, and of the three is the only one that you might find true professionals playing, although most pros would use boutique line flutes like Haynes, Pearl or Mateki.
So if you are looking for a student line flute, you would probablly be fine with any of those. For the money, you might get the most bang for your buck with a mid-line Jupiter. If you can't try them in person, my suggestion would be Yamaha, but it will cost you more.
If you are an older student or thinking of majoring in music in college, take your time and try a lot of flutes at a good music store or two... picking (or disregarding) merely upon brand is really tough to endorse. Any brand has a number of individual lines of flutes with various features, but some things to look for in advanced flutes would be:
SOLID silver (not plated or silver headjoints)
ribbed construction
french pointed arms
gold plated lip (mainly for appearance, but some people have sensitivity to the silver and it can wear from the lip)
Split e key for hight notes
There's lots of other features to be found...
happy flute hunting!!!