Converting 3 steps table to only one table using CALCULATETABLE function

Hi, I am new to DAX and I have an assignment which requires me to simplify all 3 steps table to only one table using CALCULATETABLE function in DAX.

Been trying this for almost 2 weeks now, still can't seem to get the idea.
Does anyone know how I can simplify these steps to one table only using CALCULATETABLE function?

Thank you

Table 1:

forecasting_uoa_transactions_step1 = 
    FILTER (  
        SUMMARIZE ( 
            uoa_transactions_history, 
            uoa_transactions_history[Date], 
            uoa_transactions_history[Country], 
            "UoA", SUM(uoa_transactions_history[Total Changes])
        ),  
        DATEDIFF(uoa_transactions_history[Date],TODAY(),MONTH)<variable_uoa_per_cust_window[Variable_UoA_per_Cust_Window Value]+1
    )

Table 2:

forecasting_uoa_transactions_step2 = 
    SUMMARIZE ( 
        forecasting_uoa_transactions_step1, 
        forecasting_uoa_transactions_step1[Country],
        "UoA Per Customer Average", AVERAGE(forecasting_uoa_transactions_step1[UoA_per_Customer])
    )

Table 3: 

forecasting_uoa_transactions_step3 = 
    SELECTCOLUMNS ( 
        customer_transactions_forecast,
        "Date", customer_transactions_forecast[YearMonth],
        "Country", customer_transactions_forecast[Country],
        "Customers", customer_transactions_forecast[Total Headcount]
    )