] Obviously she means Bingley and Darcy as the first two beaux, but who is the third - Mr. Hurst? If so, the word "beau" obviously has a much looser meaning for her than I would have thought - not "potential husband", but "any male dancing partner"! ;-)

I think Caroline is being a bit cute here -- she has obviously seen that her brother is very attached to Jane, but she wants to make it seem that that is not the case. By calling all three men "beaux," including Mr Hurst, she is implying that none of the Netherfield party males were ever more than dancing partners. I wonder how long it took her to think of that line for her letter? Did she mentally draft several versions before she arrived at that particular phrase? While I have never been impressed with Caroline's quickness, I do think her capable of being clever when it suits her purpose -- most especially when she can be a bit catty about it.