if object_id('tempdb..#t', 'U') is not null
 drop table #t;

create table #t (s char(30), f1 numeric(18,3), f2 numeric(18,3), f3 numeric(18,3), f4 numeric(18,3));
insert into #t (s) values ('1'), ('aaa'), ('5.5'), ('6,6');
go

update #t set f1 = case when isnumeric(replace(s, ',', '.')) = 1 then replace(s, ',', '.') else 0 end;
go

update #t set f2 = case when isnumeric(replace(s, ',', '.')) = 1 then replace(s, ',', '.') else '0' end;
go

update #t set f3 = case when isnumeric(replace(s, ',', '.')) = 1 then replace(s, ',', '.') else cast(0 as float) end;
go

update #t set f4 = isnull(try_cast(replace(s, ',', '.') as numeric(18,3)), 0);
go

select * from #t;
go