Let's say there is some number like 12345. In Konglish (Korean English), I can label 3 as the hundred's position number and 5 as the one's position number. I don't think this is correct English. How can I label each number according to its position correctly?
You would say place instead of position, and you would drop the number entirely:
3 is in the hundreds' place
5 is in the ones' place
You could go on in the same way with all the other places: '1' is in the ten thousands' place, or in the number 7.86, 8 is in the tenths' place.
Another alternative is use the word "digit":
3 is the hundreds' digit.
5 is the ones' digit.
The term for this is "ordinality". The ordinal number of
12345 is "second" because the sequence is read from left to right.
The terms ones digit, tens digi, hundreds digit, thousands digit, etc... are also correct (probably more so for what you want), though the meaning is different.