Your statement was:

"A RAID0 over three disks has about 1/3 the MTBF of a single disk."

This is incorrect, the MTTF and MTBF are not significantly changed. Assuming you meant failure probability, my issue with the probability variance is the linear relationship you imply.

If the variation were linear, a RAID array composed of drives with a 5% failure probability would reach certainty of failure (1.00 probability) within the interval at 20 drives. In actuality, it takes 225 drives to reach that probability.

The difference is a real world consideration for capacity management. What it means is that RAID0 arrays are not as failure prone as people think they are.