I've experienced the following:
WW4M
WT4M
TT4M

Of the three, the best one was with two trans - but honestly, none of these measured up to a true porn star experience. The two girls was pretty memorable because they were twins (one was a surfer and the other a goth), and the girl/trans episode was like a Halloween nightmare, a total rip-off waste of time. I have a feeling that the best experiences are with amateurs where there is lot of social lubricants (booze, weed, etc.) flowing. Occasionally, I will see ads placed where it's a husband/wife and the husband likes to dress. While I'm sure this scenario has the potential to be a freaky deaky MTF menage, you would have to be cool with cross dressing, which I am not.  
You might wanna just place an ad and see what turns up. Probably would be easier than scouring the ads. Good luck!

I love having FFM threesomes with my cisgender friends and have over a dozen that I work with.

I had an INCREDIBLE threesome with Karen Cummings  (las vegas) and my girlfriend!  She came to our room and for 90 minutes we pretty much covered it all!  Including my girl getting DPd  by us!   VERY HOT! And highly recommended!