Will be in Tokyo early September. Plan to travel by train from Tokyo to Kyoto and Kyoto to Hiroshima. Looking at cost of Japan Rail Pass suggests I could save by purchasing point to point tix.
Is it better to buy train tickets here in the USA before departing or purchase once we land Tokyo?? Will have 4 nights in Tokyo if I need to allocate time to go to ticket office. Or, what is the best website / app available to find / purchase point to point tickets online before leaving?

How many tickets do you need? Early September is not a busy travel time. For just one or two passengers you can just show up at the station and get the next train or the one after that. They are frequent, departing every 15 or 20 minutes, and high capacity. Or, you will probably be using a JR station at some point in the first few days and could buy then.

Looks like klook.com has a discount. You could also find a package with one-way ticket plus hotel.

Shinkansen Tickets (Bullet Train) - Book Japan Train Tickets Online | Search for Affordable Japan Rail Tickets - Klook US

japan-guide links this site: Tokaido Sanyo Kyushu Shinkansen Online Reservation Service! (smart-ex.jp)

Shinkansen (Japanese Bullet Train) (japan-guide.com)

I used the smart-ex app and booked in advance for my 6 travellers (with large suitcases), but I likely could have booked same day as there were so many trains from Tokyo-Shinagawa to Kyoto. Not sure about Hiroshima. It's been a decade since I went there and I can't recall what I did then. (NOTE: if you use the app, be careful not to select Tokyo-Shinagawa Station if you want Tokyo Station, and vice versa. I remember the station name being truncated on app so you have to be careful.)

I had family with 2 large suitcases, the rest of us had carryon size. So I reserved ahead for oversized luggage, and seats in the same carriage as where the luggage would be (in the back of every other carriage). Our train to Kyoto was so roomy that our carryons fit easily in front of our knees, plus there was overhead shelves where you can put carry-on size luggage.

Have a good time in Japan!