We go to Japan often, have lost count of the number of times we've gone there, and based on our experience, I think you are worrying needlessly about, well, pretty much everything.
We were going to use credit cards but we've read that Japan is really not credit card friendly.
Hogwash. I suggest you consider any further advice from that source as questionable.
Credit cards are widely accepted in Japan. Especially in businesses that cater to tourists. Like anywhere else, you might find the occasional small shop or street food stall that is cash-only, but that will probably be very few places you might need. Sure, you'll want to get some cash for those odd times, but ATMs are widely available. As noted above, the Japanese postal system has post offices everywhere, and most have ATMs, and IME those work just fine with your home-town bank's ATM/Debit card. Just seek out a post office, find the ATM, and pull some cash from your bank account at home. There are also ATMs in the major airport you are probably landing at, although only some of those machines will work with US ATM networks. Look at the symbols on the back of your ATM/Debit card...look for those symbols when you come across a Japanese ATM. Easy, peasey, Japan-esey.
there is a $300 per day withdrawal limit and $900 each would definitely not be enough cash...
You really do not need to (and should not) be walking around Japan with your pockets stuffed with cash. Use credit cards for most things, certainly for hotels and any expensive restaurants. Use cash for things only when you have to - like for an ice cream cone or some fish on a stick at a street vendor. On our many trips to Japan, I don't think we've ever used that much money in cash across 3- or 4-week trips. For three days?! No way.
if we exchange US dollars for yen here in US, bank told us that there would be a 10% converting exchange rate for changing US dollars into yen and also a high rate for converting back any unused yen...
Of course. Never, ever "exchange" money - it's 100% unnecessary and a complete waste of money. Especially so for converting it BACK after your trip. Just pull $100 in Yen from your home bank account once you've landed, use credit cards for everything...you'll probably go home with $90 of your Yen unused.
We are off to Japan again in November. We will bring along some Yen we have from a previous trip, but if we didn't have any, I would not bother getting any before we get there...and we might not bother with cash at all. If that makes you nervous, then just grab that $100 in Yen from an ATM after you get there.
Japan is one of the easiest countries in the world to travel in. You will do just fine. Hope some of that helps allay your concerns.