How to select multi row in one row?

You can't combine rows this way in any of SQL implementations.
But if you have only a single field value you want to combine this way, you can use some concatenation aggregate. But as you haven't told us which RDBMS you use, it is impossible to say if you have such a feature at hand or not. Let's suppose you have SQL Server. In this case you these approaches: https://www.simple-talk.com/sql/t-sql-programming/concatenating-row-values-in-transact-sql/[^], String Aggregation in the World of SQL Server[^], http://groupconcat.codeplex.com/.[^].

Still, as you haven't specified the grouping logic (you have 6 "row3" in your imput...), the aggregate is of no use. But let's assume you want to group them by N in some order.

Assuming the solution of the latest link, and following table:

CREATE TABLE [dbo].[test](
    [number] [int],
    [id] [int] IDENTITY(1,1) NOT NULL
)

You can issue following query:

WITH MYCTE (group_id, number)as
(
    select
    ROW_NUMBER() OVER(ORDER BY ID) / 3 as group_id, number
    FROM test
)
select dbo.GROUP_CONCAT_D(number, ', ') from MYCTE group by group_id