Newton's third law emanates from the fact that the momentum of an isolated system is always conserved viz.

$$\mathrm d\mathbf p_1 +\mathrm d\mathbf p_2 ~=~0 \;.$$

From this, it can be inferred that

$$\int_{t_\mathrm i}^{t_\mathrm f}~ \mathbf F_{21}~\mathrm dt ~=~ - \int_{t_\mathrm i}^{t_\mathrm f}~ \mathbf F_{12}~\mathrm dt\tag 1$$

It could be that the two forces $\mathbf F_{12}$ and $\mathbf F_{21}$ might be unrelated without violating $(1)\;.$

However, failing to cite any evidence to the contrary, we can conclude that at every instant $$\mathbf F_{12}~=~- \mathbf F_{21} \;.$$

And that is Newton's Third Law of motion which is valid for any mechanical collisions etcetera.

But, it should be kept in mind that no interaction takes place instantaneously.

It is noteworthy to quote French:

[...] There is no difficulty as far as far as "contact" collisions between ordinary objects are concerned. But in situations in which objects influence one another at a distance, as for example through the long range forces of electricity or gravitation, Newton's Third Law may cease to apply. For no interaction is transmitted instantaneously, and if the propagation time cannot be ignored in comparison with the time scale of motion, the concept of instantaneous action and reaction can no longer be used. ...

So, the above quote explicitly makes it clear when it is valid to use the Third Law.

Newton's third law emanates from the fact that the momentum of an isolated system is always conserved viz.

$$\mathrm d\mathbf p_1 +\mathrm d\mathbf p_2 ~=~0 \;.$$

From this, it can be inferred that

$$\int_{t_\mathrm i}^{t_\mathrm f}~ \mathbf F_{21}~\mathrm dt ~=~ - \int_{t_\mathrm i}^{t_\mathrm f}~ \mathbf F_{12}~\mathrm dt\tag 1$$

It could be that the two forces $\mathbf F_{12}$ and $\mathbf F_{21}$ might be unrelated without violating $(1)\;.$

However, failing to cite any evidence to the contrary, we can conclude that at every instant $$\mathbf F_{12}~=~- \mathbf F_{21} \;.$$

And that is Newton's Third Law of motion which is valid for any mechanical collisions etcetera.

But, it should be kept in mind that no interaction takes place instantaneously.

It is noteworthy to quote A.P.French in his book Newtonian Mechanics:

[...] There is no difficulty as far as far as "contact" collisions between ordinary objects are concerned. But in situations in which objects influence one another at a distance, as for example through the long range forces of electricity or gravitation, Newton's Third Law may cease to apply. For no interaction is transmitted instantaneously, and if the propagation time cannot be ignored in comparison with the time scale of motion, the concept of instantaneous action and reaction can no longer be used. ...

So, the above quote explicitly makes it clear when it is valid to use the Third Law.

That is not true, even when influeance is at a distance, momenta do balance. The Earth exerts same pull on the Sun as the Sun on the Earth, electron on proton etc

'momenta do balance'.... hmm, sometimes, it is vexing on how misinterpretation leads to wrong conclusion. Never ever have I said, it is not conserved for an isolated system. Probably the asker couldn't get what Mr. French wanted to tell. (He also used the word may;but that's trivial).

The main point is echoed in his statement:

[...] no interaction is transmitted instantaneously, and if the propagation time cannot be ignored in comparison with the time scale of motion, the concept of instantaneous action and reaction can no longer be used.

Momentum would be conserved even then also but not instantaneously thus violating the Third Law as he explicitly mentioned.

Newton's third law emanates from the fact that the momentum of an isolated system is always conserved viz.

$$\mathrm d\mathbf p_1 +\mathrm d\mathbf p_2 ~=~0 \;.$$

From this, it can be inferred that

$$\int_{t_\mathrm i}^{t_\mathrm f}~ \mathbf F_{21}~\mathrm dt ~=~ - \int_{t_\mathrm i}^{t_\mathrm f}~ \mathbf F_{12}~\mathrm dt\tag 1$$

It could be that the two forces $\mathbf F_{12}$ and $\mathbf F_{21}$ ight be unrelated without violating $(1)\;.$

However, failing to cite any evidence to the contrary, we can conclude that at every instant $$\mathbf F_{12}~=~- \mathbf F_{21} \;.$$

And that is Newton's Third Law of motion which is valid for any mechanical collisions etcetera.

But, it should be kept in mind that no interaction takes place instantaneously.

It is noteworthy to quote French:

[...] There is no difficulty as far as far as "contact" collisions between ordinary objects are concerned. But in situations in which objects influence one another at a distance, as for example through the long range forces of electricity or gravitation, Newton's Third Law may cease to apply. For no interaction is transmitted instantaneously, and if the propagation time cannot be ignored in comparison with the time scale of motion, the concept of instantaneous action and reaction can no longer be used. ...

So, the above quote explicitly makes it clear when it is valid to use the Third Law.

Newton's third law emanates from the fact that the momentum of an isolated system is always conserved viz.

$$\mathrm d\mathbf p_1 +\mathrm d\mathbf p_2 ~=~0 \;.$$

From this, it can be inferred that

$$\int_{t_\mathrm i}^{t_\mathrm f}~ \mathbf F_{21}~\mathrm dt ~=~ - \int_{t_\mathrm i}^{t_\mathrm f}~ \mathbf F_{12}~\mathrm dt\tag 1$$

It could be that the two forces $\mathbf F_{12}$ and $\mathbf F_{21}$ might be unrelated without violating $(1)\;.$

However, failing to cite any evidence to the contrary, we can conclude that at every instant $$\mathbf F_{12}~=~- \mathbf F_{21} \;.$$

And that is Newton's Third Law of motion which is valid for any mechanical collisions etcetera.

But, it should be kept in mind that no interaction takes place instantaneously.

It is noteworthy to quote French:

[...] There is no difficulty as far as far as "contact" collisions between ordinary objects are concerned. But in situations in which objects influence one another at a distance, as for example through the long range forces of electricity or gravitation, Newton's Third Law may cease to apply. For no interaction is transmitted instantaneously, and if the propagation time cannot be ignored in comparison with the time scale of motion, the concept of instantaneous action and reaction can no longer be used. ...

So, the above quote explicitly makes it clear when it is valid to use the Third Law.