Super easy: create a pixel based custom audience from website visitors I the last 4 days. The 5th day they will be excluded from the custom audience = no ad served to them but the new ones. And if you want it to be served 3 times per week, reach is not the correct optimization, as it will show the ad only once per day to your audience. You need to play with the budget in Fb if you want to control the rwach. It is not possible to stablish a frequency or to cap it as in Google.