She also warned about frivolous questions... Mat asking whom he will marry would not have been frivolous, as it happens, but from Moiraine it would.

Much points toward the fact the Finns don't have all the answers. They can't see the Shadow, that is foreign to the Pattern; they can't have answers completely unrelated to the person involved (they seem to read your own thread); they don't seem able to give you the unimportant answers, things which aren't written in stone in your future.

It sounds indeed likely Moiraine had a question about how best to guide Rand, and she got a riddle about saidar as an answer. At least, she mysteriously (and with a smug look) answered Egwene asking in the Waste that she remembered how to guide saidar. She could have gotten there on her own, or she may have understood the riddle.

She was extremely concerned about Rand's next move, so it sounds quite possible she found a way to ask about what hers should be. This could be why she responded how she responded to the whole thing about Rhuidean or the WO.

The Finns being the Finns, Moiraine's answers would have been mostly frustrating and useless at first, until she started puzzling them out. We saw the very frustrated Moiraine in Tear, and once she was in the Waste she changed, and rather rapidly after Rhuidean.

It sounds fairly likely one of her answers gave guidance as to how she should deal with what she learned in Rhuidean.

My hunch is that her third answer is something she will be able to puzzle out only once she's rescued and learn some of what is going on. I have no idea what she asked precisely, but I think it will have to do with what the most important for her to do in the future must be...