Hi Julia,
You can do what you want by using a Lot Size equal to 3 days. In ECC you don't have this option, but you can create a custom lot size to group your requirements every 3days. Then once the materials are ciffed to APO, you will see in MAT1 this setup:
With this, your SNP run will group your dependent requirements and create a requisition every 3days.
Kind Regards,
Mariano