This kind of homework questions are more appropriate on StackOverflow, and please remember to post a MCVE question (that is, you should give us a testimonePartecipaudienza/2 subset to play with, insted of an useless ListaTestimone), but anyway…
getListaTestimoniUdienza(ListaTestimone):-
findall(N*T,
(aggregate(count, Y^testimonePartecipaudienza(Y,T), N),
N >= 3
), ListaTestimone).
?- getListaTestimoniUdienza(L).
L = [4*'http://www.semanticweb.org/luigi/ontologies/2020/2/untitled-ontology-11#Pippo_Testimone'].
Before asking more you should take some time to dig deeper into library(aggregate) and library(solution_sequences).
Above I was expecting library(aggregate) to be linked to docs, more or less as I did manually for library(solution_sequences), given the recent work about the linkify plugin.
Is my expectation unrealistic, or plain wrong ?