Teaser 3068: Valued playwrights

From The Sunday Times, 11th July 2021 [link] [link]

I have given each letter of the alphabet a different whole-number value from 1 to 26. For example, P=4, L=8, A=3 and Y=24. With my numbers I can work out the value of any word by adding up the values of its letters, for example the word PLAY has a value of 39.

It turns out that the playwrights

BECKETT
FRAYN
PIRANDELLO
RATTIGAN
SHAKESPEARE
SHAW

all have the same prime value.

Also COWARD, PINERO and STOPPARD have prime values ….

…what are those three prime numbers?

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