Hi, recently I had chance to Integrate 3PL with 365FO. However the in 3PL we cannot pushed the date so instead design was one way (D365<-3PL), that is reading inventory transactions on daily basis configured by batch job.
Step 1: Read the item codes periodically from 3PL and dump the data in D365FO, so that item codes are reconciled.
Step 2: Read transactions of inventory periodically "receipts"(+) and "sold"(-)
Step 3: Based on financial requirement for "receipts" you can generate purchase orders with product receipts posted , and based on "sold"(-) generate either sales orders with packing slip posted or inventory adjustment.
Note: allow negative posting, eventually it will be reconciled and keep these item codes unique by using different Item model group or procurement category.