I need to workout if someone's qualification has expired. Once someone has completed a course it expires in 3 years from the date of completion. If their qual has expired but they have a nominated date then that will count as being valid

What I currently have

EmployeeID     course        Completed Date        Nominated

    1           IT             2021-06-18
    2           IT             2020-10-10
    3           IT             2017-01-01
    4           IT             2017-01-01       2022-05-01

What I would like

EmployeeID     course         Completed Date    Nominated    Expiry          Valid 

    1           IT             2021-06-18                    2024-06-18       Yes
    2           IT             2020-10-10                    2023-10-10       Yes
    3           IT             2017-01-01                    2020-01-01       No
    4           IT             2017-01-01     2022-05-01                      Yes

I've managed to create the expiry column, need help with the 'Valid' case expression

cast(dateadd(year,3 ,[Completed_Date]) as date) as [Expiry]