Does that mean XMGSF do 3 splash damage or not ?

Or printed > possess > use ?

(just to know, as i'll have to judge those kind of situation soon)

Printed is not the same thing as Possess. She would only do Area of Effect damage equal to the PRINTED number of bolts.

I was under that feeling as well, but I had to check
Thank you for your answer

Does this work the same for other specials?

Galadriel can use mind control as if she had a range of 8. Can that be perplexed?

Yes. You perplex her range +1. When she uses MC the "range of 8" is a replacement value which is then modified by the Perplex to give her a range of 9 for that effect. The same can be done to Wonder Girl for Pull Them Closer.
But the AoE for EE is specifically tied to the number of printed on the dial, so while effects that allow additional will work for creating more targets they won't effect the AoE damage at all.