Typically used to describe chemicals, or your ex girlfriend

If "poisonous" are parallelograms and "venomous" are trapezoids, "toxic" would be quadrilaterals in general. (Can't use square/rectangle analogy, because squares are a type of rectangle, and venom/poison is not a type of poison/venom.)

Aside from that, there aren't too many rules on "toxic".

Poison and venom will both cause serious acute injury with the possibility of immediate death. Both can be considered "toxic".

Just to be confusing, "poison" and "poisoning" can have substantially different connotations. For example, the heavy metal "lead" would not normally* be considered a "poison". Lead would generally be considered "toxic".

But, repeated exposure to lead to the point that it causes physical symptoms is referred to as "lead poisoning".

Same thing with mercury: it would be considered "toxic"; it wouldn't normally* be considered a poison. But repeated exposure to mercury would be considered "mercury poisoning".

(* If a third party were to deliberately introduce lead or mercury into the body of an individual, the substance would then be considered a "poison".)

https://www.sciencelearn.org.nz/resources/364-poisons-and-toxins

Sounds like poisons are injested, and toxins are poisons that are produced within the body through reactions. And venom is just poison that's only harmful in the bloodstream.

As a native English speaker, uuuuhhhhhhh

Toxic can be used to refer to something that will slowly damage the body in some way.
e.g. tobacco smoke, which doesn't kill you right away, but slowly turns you into a zombie, by destroying your mental ability and cause faster ageing.
Another, is Lead (Pb), which lead to the destruction of the Roman empire.