There are different minhagim regarding the mourning preceding Tisha B'Av. Ashkenazim follow the opinion that we do not bathe, wash laundry, etc for the entire 9 days from Rosh Chodesh Av until Tisha B'Av, and the Sefardim follow the custom to only refrain during the calendar week of Tisha B'Av itself. Therefore this year when the 9th of Av falls on a Sunday, Sefardim will not observe these restrictions at all.
There is no prohibition on purchasing clothing during the three weeks, only the nine days. However, one may not say the beracha of Shehecheyanu during the 3 weeks and should not buy an item that would require this beracha.