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We know the general figure of Venn diagrams for two or three distinct sets.

There are many formulas related to two or three sets.

For example, one of Distributive Law is

$$A cup (B cap C) = (A cup B) cap (A cup C).$$

We can visualize it by using Venn Diagram, and guess that it is true.

And also, we can prove it to show that each side is contained in the other side.

It is my question. For rigorous proof, I know we should only use mathematical logic and theorem. Nonetheless, I want to check that Venn Diagram proof is also available for some easier cases.

Can Venn Diagram be one method of proof?

Can we prove that all proof by using Venn Diagram method for two or three sets is true?

If we prove that, then all statements for two or three sets can be strictly proved by using Venn Diagram.

Venn diagrams are not a formal proof, nor a substitute for it, just an illustrative tool that can be useful as a guiding tool for your narrative/proof.

If writing a formal proof for this law, you will need to show

$$A cup (B cap C) subseteq (A cup B) cap (A cup C) ;;; text{and} ;;; (A cup B) cap (A cup C) subseteq A cup (B cap C)$$

and then use the fact that if $X subseteq Y$ and $Y subseteq X$, then $X = Y$.

If you don't need formality, then in the appropriate context it can be used, I suppose. And, up to your ability to produce said diagrams, you could use a Venn diagram of $n$ circles, depending on what you're proving, but it gets messy quick so I wouldn't recommend it for more than $3$ sets.

In short, it depends on the level of formality that is expected of you. There's no denying that Venn diagrams in contexts like these are super, super helpful in illustrating concepts, and can be taken as a sort of heuristic proof, but they are not a substitute for formal proofs.

I say this in light of the assumption that you are probably encountering this in a class of some sort like a number of questions here. Classes in set theory, generally, will expect formality, not Venn diagrams, for example. In research, publications, journals, etc., things are much, much murkier depending on the context.

I think that this is a case of "consider the audience".

If the proof is intended for research-level mathematics, I can't imagine a scenario where a Venn diagram would be necessary because any statement that can be sensibly encoded in a Venn diagram is probably elementary enough to not require a proof.

If the proof is from a student studying a topic, then this depends on the teacher's expectations. If they are teaching you formal proof techniques, then a Venn diagram would probably not be sufficient because the student would be missing the point of the exercise. This might depend on the level of the course though.

If I were teaching a proof writing course to math majors, I would not consider a Venn diagram to be sufficient. If I were teaching an intro to discrete math for CS or science majors, then I would accept a Venn diagram if it was accompanied by a sentence or two explaining why it shows what the author is claiming it shows.

I would say that Venn diagrams are not good for formal proofs, but great if you have to say whether a formula is a tautology or not - just check for any counter-example and if you find one, then the formula is not a law.