data.sql

select
	t.ticketid, t.st,   greatest(t.modifiedtime, ifnull(a.dt, t.modifiedtime), ifnull(c.dt,t.modifiedtime)) dt,
	t.tec, t.title,	
	TIMESTAMPDIFF( day, greatest(t.modifiedtime, ifnull(a.dt, t.modifiedtime), ifnull(c.dt,t.modifiedtime)), NOW() ) diasabando
from
	tksabc2 t
 	left join atividades a on a.tk = t.ticketid
	left join comments c   on c.related_to = t.ticketid
group by 1, 2	
order by 1, 2;