What my comment was about was certain web sites selling what are effectively redundant motors that at one time fetched scrap value and implying that they make 400W wind turbines. You may see a peak near that twice a year but the average daily power is likely to be in the 10's of watts or lower. Your 3A at 18 mph is really quite respectable but I doubt that the average power on most days is anything near your 75W.

If you are stuck with little things then that may be your only option but even a decent 6ft machine would equal the output of 4 of those things at 18mph. Multiple small inefficient and costly machines may be your only option and if it suits you that's fine but for most people one good larger machine would be more cost effective and a more permanent solution.

If the motor comes dirt cheap then fair enough but as to spending more on them than the cost of building a reasonable alternator, that would not be my choice.

Returning to your original question, most use these things at 12v and burn out in a gale is realistically possible but way beyond your 3A. At 24v I doubt that it will ever go fast enough to burn out.