If that battery was fully charged, it shouldn't go flat in 3 miles!
If you have a battery charger, charge it up and have a look at the figures on a multimeter. It should be pulling virtually no current off and should be making 14v+ idling.
If I might suggest calling a local, mobile mechanic to take a look as a quick fix such as a dislodged wire, or even a short circuit could give the symptoms you describe.
I had a Ford Orion that suddenly started to lose all battery charge on occasion. Eventually found that the main feed to the starter motor had a patch of insulation worn off which allowed the cable to ground on the body when parked at certain angles. Could flatten the battery from fully charged in 10 minutes, and produced many sparks but impossible to see most of the time, as the cable was tucked away.