You can do this with a template search. Open the advanced search dialog, choose the template search tab, then enter this as the template:
Code:
program:
vals = globals(vals='');
if !vals then
all_authors = book_values('authors', 'authors:true', '&', 1);
for aut in all_authors separator '&':
if book_count('authors:="' & aut & '"', 1) >=# 3 then
vals = list_join('&', vals, '&', aut, '&')
fi
rof;
set_globals(vals)
fi;
res = '';
for aut in 'authors':
if str_in_list(vals, '&', aut, 1, '') then
res = 1;
break
fi
rof;
return res
The search type is number and the search value is 1.