is it true that a person could learn three or more languages at one time?  

My girlfriend speaks. Russian, Georgian, English, some French and even less German. I have no idea on how she learned Frensh. It is possible why not? If you live in Japan and you study English as a second language then move to some other part of the world. then why not?

I suppose it's possible, it's just depending on the person who wants to learn the new languages. It may be difficult at the beginning, as the languages are so foreign to us, but I always believe that the more we practice, the better we'll be. It depends on one's persistence and patience, I suppose. As for myself, I didn't learn three or more languages at one time, I even can't master my English.

I think the best way is to learn one language to a reasonable level, then move onto another, but keep practising the one you just learned so you don't forget it! If you try to learn two or more languages from scratch at the same time it becomes confusing with learning vocabulary, especially if the languages are in the same family (eg both Romance languages like Italian and Spanish). It's much better to learn a language a while until it becomes relatively fluent and you can read, speak and write with ease, before you move on, so you can simply switch your mind into "French mode" or whatever. I've tried to learn Welsh while taking an advanced Spanish class in the past and it was very confusing. When I wanted to sit down and write in Spanish, my mind would think in Welsh. I'd say stick to one at a time :)