3 meters should not be a problem. I don't know how fast the communications rate is between the ECU & the PC. It probably is not all that fast but 100 feet still seems like a stretch. But if it worked, who am I to argue. I still would not use an extension any longer than necessary.
That must be some kind of powered extender. Plain cables will not work that far. From USB.org:
USB Info: Frequently Asked Questions
USB Cables, Connectors, and Networking with USB
Q1: How long of a cable can I use to connect my device?
A1: In practice, the USB specification limits the length of a cable between full speed devices to 5 meters (a little under 16 feet 5 inches). For a low speed device the limit is 3 meters (9 feet 10 inches).
Q2: Why can't I use a cable longer than 3 or 5m?
A2: USB's electrical design doesn't allow it. When USB was designed, a decision was made to handle the propagation of electromagnetic fields on USB data lines in a way that limited the maximum length of a USB cable to something in the range of 4m. This method has a number of advantages and, since USB is intended for a desktop environment, the range limitations were deemed acceptable. If you're familiar with transmission line theory and want more detail on this topic, take a look at the USB signals section of the developers FAQ.
Q3: How far away from a PC can I put a USB device?
A3: With the maximum of 5 hubs connected with 5m cables and a 5m cable going to your full speed device, this will give you 30m of cable (see section 7.1.19 for details). With a low speed device, you will be able to get a range up to 27m, depending on how long the device's cable is. With a straightforward cable route, you will probably be able to reach out 25m or so from the PC.
The USB extender I have works up to 150 feet, but I got 100 feet of Cat5 cable. Here is the link to the USB extender I have. I don't know what to tell you, except that I have no problem loading tunes to my bike out in the drive. http://www.pccables.com/cgi-bin/orders6.cgi