Something like

select

case

when <logical expression1 depending only on High, Medium, Low columns> then 'High'

when <logical expression2 depending only on High, Medium, Low columns> then 'Medium'

else 'Low' end as [ColHML],

sum(case .... ) then 1 else 0 end) as [Column1],

sum(case .... ) then 1 else 0 end) as [Column2],

sum(case .... ) then 1 else 0 end) as [Column3]

from sometable

Group by High, Medium, Low