Considering time limitations and obvious scarcity of resources (human bodies), the most likely uses that I can see are espionage/investigation, learning (kudos @akaioi), and dirty jobs that a magician does not want to do with their own body. I am thinking about something similar to clones in the Dark Matter series, except the memory transfer is immediate and the clone does not have a physical resemblance to the magician.
Spying is an obvious use. You just need to obtain a suitable body and you are good to go for three days. However, a magician should be careful not to expose itself, since their personality is different from the original body 'owner'.
Mental cloning will not be possible to use for an extended espionage network. It will also not be possible to use it for long-term projects. Moreover, since a magician has to be in a town or within a short travelling distance from it, it might not be practical to use this magic at all except in extreme circumstances. Any well-organised security group will keep tabs on all magicians known to be capable of mental cloning.
Investigation tasks are akin to espionage, but probably involve lower risk of exposure since a magician does not need to pretend to be someone they are not. Some kind of identification codes can be used to prove to informants that yet another body visiting them is indeed possessed by a specific magician.
One mind is good, but many are better!
All learning will be facilitated and sped up. However, the effects will depend on a type of knowledge a magician is trying to acquire.
Knowledge requiring muscle memory
Skills, such as fencing, dancing, crafting, heavily rely on muscle memory, which can be described as a brain rewiring to increase the efficiency of a neural response. It is a physical process that must happen in the brain of a magician. There is no workaround for it. Observation and understanding of the underlying principles facilitate learning, but in order to acquire a functioning skill, a learner has to practice it. Therefore, do not expect your magicians to become martial arts specialists without actual sparring and training.
While the acquisition of theoretical knowledge also involves rewiring the brain, a trained magician should be well-prepared for processing and memorization of higher volumes of information. In fact, it must be a part of their training in the first place.
Mental clones will function as 'second (third/fourth/etc) selves' who will collect, process, and dump information into the magician's brain. They also can be used for brainstorming or experimental work. A mental clone will also be an indispensable fact-checker and devil's advocate (if they share memories of the original).
Clones also can be tasked to do something potentially dangerous. For example, they can be sent to negotiate with criminals, retrieve dangerous substances, or travel to unsafe places.
If a magician can secure a steady supply of bodies, they can implement a division of labour in the house. The specific details are up to a magician. But for example, a clone might be running errands or doing research while thу magician is attending a formal ceremony in the palace or travelling.
I do not think there is a potential for
- armies or anything that involves a lot of dead bodies;
- long-term projects unless they are done in a close proximity to a magician who also has a good and reliable supply of bodies;
- long-distance communications or projects that do not involve a magician travelling;
- anything that relies on special skills that a magician does not possess (assassinations, fighting, hunting, and so on);
- impersonating someone when they are in their own environment unless it is done for a very short time.