See Wigger (1885), p.172-179. It's not known how Henry III, Albert III and Magnus divided Mecklenburg between them, or even if they did it. Even if they ruled together, it's possible that they had different seats, similar to what happened in previous generations of the family. For the case of Henry III, relating to the tournament where he got fatally injured, a document from the Continuator General refers, 20 years later, as Hinricus hastiludiis intendedens in curia sua Wismer, which could or not point to Wismar as a possible seat for Henry III (and eventually his son as well), even if he died in Schwerin. For Albert, given that he married the heiress of Schwerin, it is more plausible for him to have ruled from that location. In the case of Magnus, there are mentions of Grevesmühlen as a bailiwick associated to him as property.
