Concat multiple columns in alphabetic order

Hi experts
I want to combine 15 columns alphabetically on a table and write them in another column, separated by commas. I currently do this with powershell and then write it to the sql database. But I suspect that this would be done much faster with a sql query.

column1 "D"
column2 "A"
column3 "C"
column15 "I"

Desired result:
column16 = "A,B,C,D,E,F,G,H,I,J,K,L,M,N,O"

Thanks for any help.

hi hope this helps

create sample data

drop table #Temp
create table #Temp ( col1 varchar(1) , col2 varchar(1) , col3 varchar(1))
insert into #Temp select 'B','A','C'

; with cte as 
    select * from (
                  select col1 as col1 from #temp 
                      union all 
                  select col2 from #temp 
                      union all 
                  select col3 from #temp 
                     ) a 
    order by col1 
    offset 0 rows 
select STRING_AGG(col1,',') from cte


drop table if exists #Temp;
create table #Temp ( id int identity(1, 1) not null, col1 varchar(10) , col2 varchar(10) , col3 varchar(10), col4 varchar(10), col5 varchar(10), col6 varchar(10) );
insert into #Temp select 'B','A','C','F','L','I';

SELECT id, STUFF((SELECT ',' + CAST(col AS varchar(8000))
    FROM ( VALUES(col1),(col2),(col3),(col4),(col5),(col6) ) AS cols(col)
    ORDER BY col
    FOR XML PATH(''), TYPE).value ('.', 'varchar(8000)'), 1, 1, '')
FROM #temp

Perfect, thank you