“We three” vs “us three”

we three will go to the Express mall.

You can find we/us three there, having a good time"

None of this is what a 21st century native speaker is likely to say in ordinary conversation. I have given the most likely version in bold below.

In conversation, many people do not stick to any strict rules of grammar (or indeed know any rules).

If you want a rule here's a simple one.

Remove "three" from the sentences.

We will go to the Express mall. ("We" is the subject of the verb "go")+

You can find us there, having a good time. ("us" is the direct object of the verb)"find")

Now replace the word "three" in those sentences and you have your answer.

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Having said that, this is all very artificial. I can't imagine native speakers talking like that.

We might say

or

The three of us will go to the Express mall. ("us" is correct because it follows "of")

You can find us there, having a good time. (No need to repeat "three" because it has just been said)

If you really want to repeat "three", then we would say

The three of us will go to the Express mall. (This makes "three" the subject of the sentence)

You can find the three of us there, having a good time.

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Finally, in a lower register (said by people who have never learned grammar or prefer to ignore it) you will hear

"Us three will go to the Express mall" (This goes against any formal rules but is certainly a common way of speaking informally)

"Us three" is correct.

In "we three" the meaning is "we, who are three in number". "Three" is used post-positionally and adjectivally (or in apposition) and does not influence the case change of "we" to "us", i.e. it does not prevent the change from the subjective "we" (nominative) to the objective "us".

1916 R. L. McCardell Diamond from Sky xxix. 423 You may be an adopted gipsy, and Arthur may be a born one, but of us three I am the real Romany.

I don't usually give two answers to the same question but I think it is justified in this case.

There are a few questions that come up on language forums repeatedly and that are the source of battles between participants. I recall one on Wordreference that had over a hundred pages of argument and was never resolved to anyone's satisfaction.

In the end almost all these disagreements come down to whether one is an adherent of prescriptive or descriptive grammar

Prescriptive grammar tells us how we should write or speak whereas descriptive grammar tells us how people actually speak in real life.

Answer

When asking on a forum such as Stack Exchange, you will sometimes run into one of these points of contention - this is one of them.

As a result, you have to choose between what the textbook says and what actual people say. English grammar varies in different parts of the world, so it's best to choose a particular region and stick to it.

If you want to speak like a native then you do best to choose descriptive grammar. If you want to pass a test with the highest marks then you probably need to follow prescriptive rules.

So, which are the rules to follow? The only answer in my opinion, is to discover which textbook is recommended by the people who set the test and follow that.

If you are still in doubt then come back here and quote the textbook. That way we can help you understand it.

Good luck!