Is it possible to write a SQL for all matchs in a triple join with no unions?

I’d like to perform a SQL join for the grey painted region of the following Venn diagram, but using no UNIONs:

Please, is it possible?

For the sake of simplicity, think all tables with only two columns: id and value, id is the same for all of them.

Here’s a sample data:

Set A
| id | value   |
----------------
| 1  | red     |
| 2  | magenta |
| 3  | white   |
| 4  | yellow  |

Set B
| id | value   |
----------------
| 2  | magenta |
| 3  | white   |
| 5  | blue    |
| 6  | cyan    |

Set C
| id | value   |
----------------
| 7  | green   |
| 3  | white   |
| 4  | yellow  |
| 6  | cyan    |

Resulting set
| id | value   |
----------------
| 2  | magenta |
| 3  | white   |
| 4  | yellow  |
| 6  | cyan    |

Answer

You can full join twice and add conditional logic in the where clause:

select id
from a
full join b using(id)
full join c using(id)
where 
    case when a.id is not null then 1 else 0 end
        + case when b.id is not null then 1 else 0 end
        + case when c.id is not null then 1 else 0 end
    > 1 

Not all databases support that syntax. A more widely supported approach is indeed to use union all and aggregation:

select id
from (
    select id from a
    union all select id from b
    union all select id from c
) t
group by id
having count(*) > 1