When sensing Earth's ionosphere using multiple Global Navigation Satellite Systems (GNSSs), special care needs to be taken with the receiver differential code bias (DCB) contributions to the error budget. We propose a method that is able to time-wisely retrieve the Between-Receiver DCBs (BR-DCBs) from code measurements collected by a zero-baseline setup, thereby eliminating most common error sources. We base our investigations on dual-frequency GPS (L1 + L2), BeiDou (B1 + B2), and Galileo (E1 + E5a) measurements collected in 2013 with a 30 second sampling rate by four multi-GNSS receivers of three types connected to one common antenna. For each receiver-pair, we determine the time-wise estimates of GPS/GEO/IGSO/MEO/Galileo BR-DCBs from the corresponding code measurements. We confirm that (1) the time-wise estimates of BR-DCBs for all tested receiver-pairs exhibit good intra-day stability and (2) the daily weighted average (DWA) estimates of GEO/IGSO/MEO BR-DCBs are inconsistent for receiver-pairs of mixed type, due to the presence of BeiDou code inter-satellite-type biases (ISTBs). We also identify likely factors accounting for variability in DWA estimates of BR-DCBs over a one-year interval.