A basic 3PL should provide: a receiving confirmation with unit count within 24 hours of arrival, current on-hand inventory balance by SKU on request, and outbound shipment confirmation with tracking numbers.
Better 3PLs provide a client portal with real-time inventory visibility, exception notifications when received quantities differ from expected, and daily or weekly inventory reports.
If your 3PL does not provide any of these, you are flying blind. Create a manual tracking system: maintain a spreadsheet of every inbound shipment sent to the 3PL and every outbound shipment processed. Reconcile monthly.
Inventory discrepancies should be investigated immediately. A 3PL that cannot explain missing units within 48 hours of a discrepancy report has a process problem — and it will cost you more units over time.