It didn't rate lamps at 3 hours per start but mentioned that.
It rated 28,800 hours at 12 hours a start

It also said if used on preheat approximate life is 15,500 hours.

Powell 

The rating differ according to the intended use. If the lamps are expected to be used at homes, they are rated for the 3hour switching cycle. But when their expected use is only the professional (retail illumination) sector, the lamps are rated for 12hour/start and when the outdoor public lighting, the rating is usually for 10hour/start (e.g. most HID's)

So, how many hours would my 12,000 hr metal halides and 24,000 hr mercury and HPS lamps be rated for in residential use at 3 hours per start?

It depend, to what extend is the lifetime limited by the number of starts and to what extend by the steady burning hours.
As the contribution would differ among manufacturers, you can not say it directly, you may only guess.

HID's are cold started, so electrode sputtering related wear would be significant, so we may take an assumption it's significance is in the same manner as with cold started CFL's.
So when the CFL is rated 6000hour for 2.7hour/start, it mean about 2222 starts.
The same CFL is usually rated for 5000 switching cycles, where each of them the lamp burn 15 minutes, so total 500 hour burning. Assume both starts and the running hours do the same effect, then:
Const = Starts*StartFactor + HoursBurned = 2222*StartFctor + 6000 = 5000*StartFactor + 500

The StartFactor is the starting wear recalculated to burning hour equivalent and we guessed it is the same for HID's.
Solving the equation we get StartFactor = (6000-500)/(5000-2222) = 2.47 hours/start


Assume the HID follow the same equation, then (24000hour MV case):
RatedNumberOfStarts = 24000/10 = 2400
Const = 2400 * 2.47 + 24000 = ~30000hour = ResidentialCycles * 2.47 + ResidentialLife = ResidentialLife/2.7*2.47 + ResidentialLife

The estimated life in the residential scheme would be then
ResidentialLife = 30000/(1+2.47/2.7) = ~15667 hours.

But this is only the estimation, the wear related to number of starts may well be totally different than the wear by burning hours, so based on what is the limiting factor in the "10hor/start" rating the residential life may vary from 6500 hours (when the only wear is related to number of starts) to the 24000 hours (when the only major wear are the burning hours).