Nope, that's murder. You don't have to intend to cause it for 3rd degree to qualify. Let's look at the definition:
Had it not been for the knee on the back of the neck, George Floyd would be alive today. That means that the cop caused the death. It doesn't matter if the guy caused him to die by asphyxiation or caused in part by underlying health conditions but triggered by the cops.
A knee on the back of the neck for 8 minutes is "eminently dangerous to others".
And "the depraved mind, without regard for human life is also present" is shown by the knee being on the guys back even after the guy was no longer responsive. It might be that the jury won't convict for 3rd degree because juries defer to cops, but the guy is clearly guilty.
@Doomsought posted a video that goes into detail about it and is well worth a watch: