It's doing good for just having 3 sonars, but realistically, you need multiple different types of sensors that all complement each other. Mechanical bumpers or feelers as a last resort. IR proximity detectors for close in. Sonars for further off detection. Everyone talks about how good sensing is "the" most important aspect of robotics, and how sonars alone are too prone to give errors, due to multiple reflections, grazing incidence angles, etc.
Joe Jones has a really good book [and cheap] that goes into different sensor issues in great detaill.
Some robots detect obstacle collisions by measuring motor currents, but your rover has so much torque, that probably won't work, and you need bumpers, IR prox, etc.
You might also checkout dpa's robots. He's one of the best builders. The rover isn't the best for suspension compliance, but chekout j-bot.
http://www.geology.smu.edu/~dpa-www/myrobots.html