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