Wouldn't it be a bit easier... a lot easier to stick to languages that fall within the same family or at least have something, ANYTHING, in common? I don't say this to deter you but the languages you've picked are all very difficult and Russian has nothing even remotely in common with the other two, afaik. I don't know anything about Japanese or Mandarin but, after looking through Wikipedia, at least Japanese uses a lot of loan words from Chinese which, I imagine, will make it a somewhat easier to understand the two when you start to dig into etymology for a broader understanding. The point is, I think you should scratch either Russian or Japanese AND Mandarin from your list. Learning both Mandarin and Japanese will take a long time (lifetime) but at least there's something in common between the two (loanwords, beyond that I have no idea if there's anything in common at all) even if it's small... Unfortunately I imagine (because I genuinely have no idea) that the similarities between Japanese and Chinese are about as close as Russian is to English so... good luck! but at least there's SOME relationship, something that might reinforce your knowledge of one language with the other, no matter how small.