Electrical – Is it safe to use a 100 watt LED bulb in a 3-way lamp that specifies using incandescent bulbs only

AFAIK, bulbs with a GU-10 base are driven directly from electrical mains, in which case it's OK to use a lower-power bulb. Your fixture won't reach the temperatures that it's capable of withstanding.

The only place that I know of that it might not be OK is with low-voltage (usually 12V or 24V) lighting systems such as track or task lighting: some transformers have a minimum power output that they like to supply, so if you use lower wattage bulbs, you should use more of them to compensate (somewhat defeating the purpose). If this applies, you would have an entry for Minimum power on the specifications for the lighting system.

You will be OK as far as not overloading electrically. You need to adhere to the most restrictive labeling, the 40wx3. Since you are using 27wx3, you are OK. The 75w limit is for the socket base only. It can handle 75w, but the rest of the fixture cannot. Be aware that 81 watts of fluorescent light is WAY more than the 120 watts of incandescent. If 120 watts was adequate, this could be overkill. There are various CFLs available in a range of light output, so you should be able to find an appropriate output.

Another consideration is weight. Some CFLs are rather heavy, and that weight cantilevered out by the adapter adds significant leverage on a socket only intended to support a small candelabra lamp. Multiplied by the fan vibrations, this could be an issue. Carefully evaluate how sturdy the sockets are mounted before proceeding with this change over.

And of course there is no way the bowl will fit over this sort of configuration. You will have the joy of everyone being able to see your handiwork :)