Can't you use the same timer for all sensors, since the sensors will be sampled one after the other? For each sensor you need to reset timer, start timer, stop timer, store to variable. What if you are trying to read several Ping sensors at once? You'll send the trigger to all sensors, reset timer, start timer, then monitor each sensor for pin change (interrupt?) and store that timer value without stoping the timer. After all sensors are sampled, process the data. You think it'll work? It's kind of similar to the simultaneous servo pulse generator.