If you mean that col_1 is more relevant than col_2 and so on, then :
select *
,case when col_1 like '%$keyword%' then 1
when col_2 like '%$keyword%' then 2
when col_3 like '%$keyword%' then 3
end as [priority]
from table_name
where col_1 like '%$keyword%'
or col_2 like '%$keyword%'
or col_3 like '%$keyword%'
order by [priority]
If you meant with the most column match then :
select *
,(case when col_1 like '%$keyword%' then 1 else 0 end) +
,(case when col_2 like '%$keyword%' then 1 else 0 end) +
,(case when col_3 like '%$keyword%' then 1 else 0 end) as [priority]
from table_name
where col_1 like '%$keyword%'
or col_2 like '%$keyword%'
or col_3 like '%$keyword%'
order by [priority] desc
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