There have been some reports suggesting that ANA might be willing to through-check your bag across separate NH => UA tickets. When checking in with NH, I would show the agent a printout of your UA ticket, and ask about through-checking. You'd still have to claim your bag and undergo Customs inspection at IAD, but you could then just drop your through-checked bag at the bag drop past Customs, instead of having to stop at a UA ticket counter. That would save you a few minutes.

Even without through-checking, I would think that 3.5 hours is enough time to make your self-transfer, but I do not have personal experience arriving IAD from abroad.

Good luck!