It's to give the jury a choice. If they don't think he's guilty of 2nd degree murder (which he certainly isn't, if you read the law), then they can consider third degree murder. If they don't think he's guilty of 3rd degree murder, they can still convict him of manslaughter.
Each charge has a specific legal definition. It’s not like you can say, well, he has to be convicted of something, and 2nd degree murder doesn’t fit, and manslaughter doesn’t fit, so we’ll just go with 3rd degree murder.
It’s possible to be found guilty of any one of the 3 and not guilty of the other 2. The 2nd degree murder charge requires a conviction on the 3rd degree assault charge though. Because you don’t get to 2nd degree murder without an underlying felony in MN law. So if the assault charge is tossed (either because Chauvins conduct didn’t rise to that level, or is considered protected from charge because he was a cop), the 2nd degree murder charge has to go with it. 3rd degree murder in MN is basically “depraved heart” murder. Fire your gun into a crowd and kill someone at random kind of thing. It is not for conduct directed at a specific individual. Hence why it was originally stripped from the list of charges. Why the judge reinstated it is beyond me.
2nd degree manslaughter is probably the best bet for a conviction, but still pretty shaky. That relies on the defendant intentionally creating an unreasonable risk of death. In essence, this is the “but for” explanation of the prosecution. “But for Chauvin’s restraint of Floyd he would have survived.” The defense for this is basically “but for Floyd’s extreme fentanyl intoxication and cardiovascular disease, and delay in arrival of paramedics due to the hostile crowd, he would have survived Chauvin’s restraint.”