The rear fog lamp set-up uses 21 Watt bulbs as the rear brake lights providing the same amount of shine from behing. An H3 would definitely fry the plastic hub and lens.
Having the fog lamps on is like constantly having your brake lights on, this is what provides the additional encandence for foggy conditions. In some countries it is against the law to drive with your your rear fog lights on when there is no such condition. In my country, because tailgaters are a nightmare, almost everybody who has rear fog lights uses them on at all times at night.
I would check with your local traffic autorithy to see if you may use rear fog lights. If not, hook it up as additional brake lights (the better they see you from behind... ) Or, just as additional night driving lights (using 12W bulbs).
The problem of the spring in the empty housing can be easily solved by a car electrician. In my E320T I just ordered the housing for a right-hand drive vehicle and exchange it for mine (of course, it came with the rear fog at the opposite side).