[Grammar] A,B, and C each have been....

[Snappy]

Osaka, Nara, and Kyoto are historic areas of Japan.

Osaka has once been the ancient capital of Japan. Kyoto and Nara, too.

Then is it possible to say, "Osaka, Nara, and Kyoto each have once been the ancient capital of Japan."? Otherwise, is there a better expression?

 

[bhaisahab]

Osaka, Nara, and Kyoto are historic areas of Japan.

Osaka has once been the ancient capital of Japan. Kyoto and Nara, too.

Then is it possible to say, "Osaka, Nara, and Kyoto each have once been the ancient capital of Japan."? Otherwise, is there a better expression?

"Osaka, Nara, and Kyoto are all former capitals of Japan".

 

[Raymott]

Osaka, Nara, and Kyoto are historic areas of Japan.

Osaka was once the ancient capital of Japan. So were Kyoto and Nara.
Osaka has been the capital of Japan. So have Kyoto and Nara.

The adverb "once" counts as a definite time in the past for the purposes of tense.
"Kyoto and Nara, too" is very informal.

Then is it possible to say, "Osaka, Nara, and Kyoto each have once been the ancient capital of Japan."? Otherwise, is there a better expression?
"Osaka, Nara, and Kyoto have each been the capital of ancient Japan."
"Osaka, Nara, and Kyoto were each once the capital of ancient Japan."
"Osaka, Nara, and Kyoto have all been capitals of ancient Japan."
"Osaka, Nara, and Kyoto have all been ancient capitals of Japan."
"Osaka, Nara, and Kyoto were all once capitals of ancient Japan."

R.

 

[Raymott]

"Osaka, Nara, and Kyoto are all former capitals of Japan".

Yes, that's probably the easiest way to express it - and possibly the best.