If you have R1 tires/rims, they can be configured to (up to) a 68.4" rear width. My 3720 came with R4s. It was so unstable on slopes and banks that I found myself up on 3 wheels way too often. Reversing the rims provided almost useless width gains. Over half a ton of liquid ballast on all 4 corners helped some, but ultimately it was 8" of additional rear width that finally established a working degree of stability. It's what I needed to achieve the same 68" of rear width you can get with R1s. Given the supportive construction of my rear extensions, I'm now reconsidering front wheel extensions as well.
R3s are less common, but - without extensions - are unlikely to provide any more stability than R4s. The unstable 3x20 series has been discussed in detail here, try putting the site search engine to work.
//greg//