Question

Is it possible to add a third fill color in a node?

Comments

No, since there is currently no usage for a third color ;-)
For what do you want to use the additional color?

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I need to show that some of my nodes below to three different groups, each one of them being characterized by a different color

Answer 1

You could try to add additional labels that have no text but different colors. If you use "Fit Node Width" or "Fit Node Height" as "Size" and non-zero "Insets", you should be able to size those labels to desired values.

(Corresponding GraphML)