SELECT TOP 1

AVG(main.age) AS _Average,min(main.age) AS _Min,

CASE WHEN %vid = count(main.age)/2 THEN main.age else 0 END+MAX(CASE WHEN %vid = count(main.age)/2 THEN main.age else 0 END) AS _Median,

max(main.age) AS _Max

FROM ( SELECT TOP all a.Age FROM Sample.Person a ORDER BY a.Age ) main

I am not sure there is a logical way for those tables to be joined. But for syntax purposes here is a query that works although, as mentioned,  illogically. 

SELECT
* FROM (SELECT * FROM Sample.Employee e LEFT JOIN Sample.Company c ON c.ID=e.Company) g
JOIN Sample.Vendor v ON g.Company=v.Vendor