It is my understanding that sadly, no, three-buckle are not technically proper wear for ladies in the hunt field. (from someone in-the-know at the The Middleburg Tack Exchange, VA)
They are acceptable for gentleman members wearing ratcatcher.
That being said, I have seen them on ladies in the field, but it has only been very, very rarely. They were, however, quite striking in dark, gleaming mahogany. Made a lasting and positive impression, (perhaps because they adorned a pair of lovely and long slim legs which I could not help but envy in comparison to mine).
HOWEVER, times, they do change, and in this new century, I believe that if one were to make inquiry thru the proper channels of their hunt, such as the MFH etc., respectfully asking if they were permissable ratcatcher for the fairer sex, I would not be surprised to hear that some would happily allow them.
And I fail to see where doing such would cause any harm to be visited upon anyone.