I don't know anything about that kind of stuff, but I don't see why it would matter if you're looking for sex with the female too? Or I guess if you were only looking to sleep with the male I would get it. If you're looking to sleep with them both I don't know why it would matter. Then again, I have not ever looked at this type of thing or been involved in those things so I don't know anything anyhow.
Craigslist is way too sketchy, and you don't really know who you're dealing with. Okcupid is free, there are pictures, and there's a lot more information. Plus, it even gives an option for you to put if you're single, seeing someone, or available.
I've actually posted on Craigslist as a bisexual female looking for a woman (I'm married), but not for a threesome. Mostly I found that Craigslist girls were very sketchy and irritating themselves. So while I don't find the posting annoying as I've done it myself, I find most people on Craigslist annoying in general.
I agree that OK Cupid is a much better place to find like minded women that are not sketchy. Tons more info and it seems a lot safer than a two line post on Craigslist where you really don't know what you're getting. Craigslist seems more for one time hookups where OK Cupid is more for relationships from what I've experienced.
I only use the housing section (met enough sketch balls through that, honestly) but there is a mw4w section, which is specifically for couples who are looking for a threesome, so couples should use that, even if it's the woman posting on their behalf. If it's a bisexual woman looking for another woman, I don't see why they shouldn't use the w4w section.