Getting errors when I try to group by CTEs
In your first CTE you grouped by Object_Name and all of the other entries were contained within an aggregate.  In your last CTE that isn't the case.

    SELECT c3.Object_name 
        ,c1.TimesExecuted/c1.CountExec as New_new_result
        ,c1.MinDate


I've removed the commented lines for clarity, but you'll notice that none of these are contained within an aggregate. (MIN, MAX, AVG, etc).  If you want to group by c3.Object_name you'll need to put something around the other fields.  For example:

    SELECT c3.Object_name 
        ,SUM(c1.TimesExecuted/c1.CountExec) as New_new_result
        ,MIN(c1.MinDate)