Three tables with some matching data and some not - want to data from all three
Not sure if I'm completely following but maybe this is what you are looking for?

I'm taking all three tables and then using UNION ALL (combine but don't remove duplicate records) to convert them into a single table, placing NULL's in the columns that are not shared. Then I'm taking that consolidated table and grouping on your two common columns (JobNumber, CostCode). I'm not sure what kind of data you're looking to get out of the other three columns...are you looking for the COUNT of records from each table for that JobNumber+CostCode? Or maybe the SUM?

So this sample query, will give you a unique list of JobNumber+Costcode, and the Sum of Costs, Budgets and CommitedCosts for each one:

SELECT x.JobNumber, x.CostCode
    , Costs = SUM(x.Costs)
    , Budgets = SUM(x.Budgets)
    , CommitedCosts = SUM(x.CommitedCosts)
FROM (
    SELECT JobNumber, CostCode
        , Costs
        , Budgets = NULL
        , CommitedCosts = NULL
    FROM Costs
    UNION ALL
    SELECT JobNumber, CostCode
        , Costs = NULL
        , Budgets
        , CommitedCosts = NULL
    FROM Budgets
    UNION ALL
    SELECT JobNumber, CostCode
        , Costs = NULL
        , Budgets = NULL
        , CommitedCosts
    FROM CommitedCosts
) x
GROUP BY x.JobNumber, x.CostCode