Aquí hay un ejemplo con fechas.
create table t1(created date not null);
insert
into t1(created) values (date '2011-01-09')
,(date '2011-01-10')
,(date '2011-01-11')
,(date '2011-01-12')
,(date '2011-01-13')
,(date '2011-01-14')
,(date '2011-01-15')
,(date '2011-01-16')
,(date '2011-01-17')
,(date '2011-01-18')
,(date '2011-01-19')
,(date '2011-01-20');
select floor(datediff(now(), created) / 3) * 3 as days_ago
,min(created)
,max(created)
,count(*)
from t1
group
by floor(datediff(now(), created) / 3);
+----------+--------------+--------------+----------+
| days_ago | min(created) | max(created) | count(*) |
+----------+--------------+--------------+----------+
| 0 | 2011-01-18 | 2011-01-20 | 3 |
| 3 | 2011-01-15 | 2011-01-17 | 3 |
| 6 | 2011-01-12 | 2011-01-14 | 3 |
| 9 | 2011-01-09 | 2011-01-11 | 3 |
+----------+--------------+--------------+----------+
4 rows in set (0.00 sec)
