I own both the DVDs you mentioned, and have often used The Three R's in class (I teach an 8-week natural childbirth series, and show that one around class 6, when they're very used to the sights and sounds of labor). It usually goes over just fine with some explanation. I also distribute a handout on The Three R's from Penny's web site. However, I think this video is getting to be quite outdated, and I haven't used it in the past few series.
As for Comfort Measures, it is an extremely long video, but you could use clips at various classes. I bought it to use as a doula trainer and for my own growth. I don't use it in my CBE classes, as I prefer to demonstrate and then have them try comfort measures one at a time, hands-on, so that they can feel it in their own bodies and I can help guide them on technique.
If you have a chunk of money to invest, the best use of that, IMO, would be to buy "Understanding Birth" from Injoy. On this one DVD you will have a very nice, natural, hospital birth story, stages of labor, comfort measures, and a section on cesarean birth. This one video could be used in 4 separate sessions! They do offer a discount for independent educators. There is a new revised version just out this year, and I heard about a month ago that they still have some copies of the "old" version, which they're selling at 1/2 price. That would be a good deal! I own both the old and new versions, and feel that the old one would still be very useable in class, it is certainly more updated than the Three R's.
Also, there is a Childbirth Supplies Swap Yahoo group, that can be a nice place to find educators who are selling DVDs and other teaching tools.