I have to disagree, based on experience. Remember he said a "garage" shop as in home shop. If only one person is using the machines, then normally only one machine will be on at a time. A compressor will be intermittent. And most home shops do NOT have 10 HP machines, mine certainly does not.
My house has two central AC units, clothes dryer, two cook tops and an oven for the heavy loads. When I retired and started to move my shop into the garage I found that the biggest restriction was the number of available breaker locations in the house's original panel. It had only about three or four empties and even with double circuit breakers, that just would not do. I wanted to add two 230V and about eight 115V circuits for it. There was only a single duplex outlet existing in the garage and none in the adjacent storage room where I planned to put my compressor. One more thing is I have been changing the lighting in the house to CF and later to LED so that saves some power. BTW, there is an additional benefit in any power savings in more efficient lighting in the summer as the AC units do not have to work as hard to remove the heat generated by the inefficient bulbs. Here in south Texas, with a large house like mine, that can be significant. I have also replaced 1 1/2 of the AC units with modern, more efficient units.
I did bring in a professional electrician and had him change out the breaker box for a larger one. This is OK by the code if there is a reasonable expectation that you will not exceed the Amperage of the main breaker, which in my case is 125A. If you want more at that point, then you need to replace all the wiring from the pole to the breaker box with heavier gauge.
Anyway, he replaced the breaker box in one morning and I had all the circuits I needed. I had him bring them out to boxes adjacent to that new breaker panel and I have run the circuits from there to save $$$. I have 230V for the AC unit in the window (with heat) and for the mill, 20 Amps each. I ran four 115V, 20 Amp. circuits around all three walls and alternated circuits in each outlet location. That way I have all four of these utility circuits available on each of those walls. Three more 115V circuits were run across the ceiling for the benches and equipment in the middle of the room: the lathe and the big drill press each have their own, private circuit. Finally one circuit was run to the compressor in the storage room where the breaker panel was. I also added a couple of outlets in there on the above circuits so I can eventually locate my bench grinders in there. I still have one or two breaker slots available.
In several years of use I have not had a single power problem either in the house or in the shop. I was a bit worried about running things in the shop while the clothes dryer or the oven was in use, but that has not materialized.
I mentioned that my main breaker is 125 Amps. If you total up all the breakers in my new box it will probably be three times that much. But, as I said above, this is OK with the code if it is based on a reasonable pattern of expected usage.
As for the three phase equipment, a VFD (Variable Frequency Drive) is the way to go. It allows good speed control and it is probably the most efficient way to do the conversion. So, it will use the available electricity in the most efficient manner: in other words, it will run your three phase machines with less electricity. It is probably a good idea to have dedicated breakers and circuits for each VFD, even if you will probably only use one at a time. With limited power available I would pay close attention to sizing the VFD and to the other specifications.
100 amps at 240 is pretty limiting. You can run a 10hp Phase Perfect on 60-70amps to give you 30 amps 3 phase 240 with decent overload capability at machine startup.
Don't forget heat, lights, compressor, etc..
