What is 3rd degree murder ?

In what country or state?  They all define it in their own way, if they have degrees at all.  And it's not manslaughter or it would be called manslaughter.

5 yrs tops......................

In Mn the difference is the state of mind.

609.195 MURDER IN THE THIRD DEGREE.

(a) Whoever, without intent to effect the death of any person, causes the death of another by perpetrating an act eminently dangerous to others and evincing a depraved mind, without regard for human life, is guilty of murder in the third degree and may be sentenced to imprisonment for not more than 25 years.

609.205 MANSLAUGHTER IN THE SECOND DEGREE.A person who causes the death of another by any of the following means is guilty of manslaughter in the second degree and may be sentenced to imprisonment for not more than ten years or to payment of a fine of not more than $20,000, or both:

(1) by the person's culpable negligence whereby the person creates an unreasonable risk, and consciously takes chances of causing death or great bodily harm to another; or....

The exact statutory definition of third-degree murder is "[t]he unlawful killing of a human being, when perpetrated without any design to effect death, by a person engaged in the perpetration of, or in the attempt to perpetrate, any felony other than" nineteen enumerated categories of felonies.

If that was the charge to the jury, the jury would have to find innocent, since the copper was doing his legal duty under the law. Thus, a prosecutor would have to find some other charge(s) to make sure the copper hangs, to suit the mobs will.