Condition column based on sales and date

Hi experts,

I just start shifting gears from Tableau to Power BI. I came across a problem which I need to add a condition column for every customer based on their sales by month.

You can download the Power BI file Here

Here's the conditions:

(current month is July)

1. If the customer sales of current month is zero or null, then the condition shows "Not Buy"

2. If the customer sales of current month is less than previous month and is also less than the month before previous month, then "Down 2 months" (less than each of the previous month sales, not sum of the previous month sales)

3. If the customer sales of current month is less than previous month sales and is greater than the sales of the month before previous month, then "Down 1 month"

4. If the customer sales of current month is greater than previous month sales, then "Good"

Thank you for your help.

Sherman