Hi,
often I need to set the three Colour Gradient colors with the same value.
Is there a feature that once selected the first color sets other two colors?
No there isn't
Well sort of...
As you are not using the sectors, set them like below, and all 3 sectors will become 1 and therefore be the same colour, which is the first one selected (Green).

Thank you Paul for your help,
Nicolò