Will take only three train trips; cheaper to buy separate train tickets or pass?

Dear Admin,

We are a family of two adults and two kids (8 and 13). We will soon need to travel to these destinations in Japan:

Osaka to Kyoto on August 10, 2018. Our plane lands in Osaka at 17:50 so we will need a train in the evening.

Kyoto to Tanabe on August 13 in the morning.

Katsuura to Tokyo August 19 in the morning

Will it be cheaper for us to buy the Japanese rail pass in advance or should we simply buy tickets at the train stations?

I have read that train tickets are bought at train stations, not online. Is there a danger of us not being able to get a place on our preferred train if we leave it to the last minute?

I look forward to hearing from you.

Kind regards,
Kasia Riding