Correct spoken english

[kossi]

Would you say

she is the eldest of 3? or

she is the oldest of 3?

[kempie]

I believe that if the 3 are siblings then 'eldest' is the preferred form, whereas oldest does not have any implied relationship between those being described.

[Emily Ball]

Spot on kempie.If she had only one sibling then she would be the 'elder' of the two.

[Dusky]

I understand that elder & eldest developed pretty much as euphamisms for older & oldest for use when talking about people, since it is deemed impolite to pass comment about someone being "old".  Just as it has always been extremely rude to ask a lady's age.

[Dusky]

The "er/est" thing applies all over the place.  If you are talking about two things, you should always use the suffix "er" ("the lesser of two evils" or "the later of two trains") but once you're talking about three or more, you should use "est" (except, of course, you can't say "lessest"; that would have to be "least" if there are three or more evils).