I have a Thule one which goes behind the tow bar - it works well and can carry up to 4 bikes. I find it difficult to get the bikes on and off without scratching them though (the carrier has two arms which come out, which you slide the bikes over). Thule do a 3 bike one as well, which has tracks for the wheels to go on - I think this would be more satisfactory. I also have a Thule indoor carrier which is way better as far as I'm concerned (for security, safety, economy and keeping the bikes scratch free), but obviously at least 3 seats need to come out, depending on how many bikes need to be carried...