Wow, is there ever a lot of misconceptions and misunderstandings surrounding this....
I'll take some pictures today once it gets light out to show how it's typically done around here.
Someone else has noted that it's normal to take the three phases and neutral down the whole street and tap different phases at different points for different building's supplies. It's quite possible that your next door neighbour is on a different phase - something to bear in mind if you're having a party in the garden and someone decides to throw an extension cord over the fence to power the disco lights from the neighbour's mains, while you power the music from your house's mains.
Wait... What?!!
Why would it matter if you use extension cords from mains supplied from different phases? You're not connecting anything
across the phases or shorting them together.
If your neighbor is on a different phase, it is no different than having two outlets in your own home supplied by different phases in all those homes over there that
do have three phase.
You're missing the fact that the OP is in North America, where a different system prevails. Rather than picking off individual phase+neutral pairs for each property, there's a centre tapped transformer, typically every 1-3 houses (primary connected across two of the three phases), usually on a pole.
Uhh, no. First of all, the pole transformer is connected from one phase to the grounded neutral,
NOT across phases. Most residential streets only have
one phase strung along them. The feed lines
between those streets that come from distribution will have all three, and individual streets are connected to different phases to balance out the load as seen from upstream.
While perhaps in some very rural setting you would have your own transformer for one house, here in the city it is typically more like 4-10+ houses per transformer when it is overhead and probably 20+ houses if it's underground buried lines. Small towns with larger properties (like my place at the lake where it is half an acre, the pole pig is shared between me and the house across the street) might share a transformer between just two or three properties, but that would be ridiculous in the city.
This supplies two phases at 180º from each other and a neutral. Each phase is nominally 120V with respect to the neutral and 240V with respect to each other.
Correct.
It's normal to bring two phases and a neutral in from the transformer, and bond the neutral to earth at the distribution board. Hefty appliances get a 240V supply, others 120V.
Almost correct. It's not
two phases, it is a single centre-tapped phase, with the centre-tap grounded, often called "split-phase".
So, three phase is a wholesale change of supply in North America, not just bringing in the missing phases. You'll need a 3 phase transformer in place of the existing two phase.
That depends. If you're near something commercial, there will be three phase right there and it is just a matter of bringing in the other phases, although most commercial stuff or multi-unit residential units have their own dedicated transformers, sometimes on the commercial site itself, with the transformer owned by the customer, otherwise three pole mounted or a ground-pad-located transformer-in-box owned by the utility so you'd still be on the hook for some sort of transformer-installation-charge (even if you don't have to actually pay for the transformer itself) if you're using the power company's transformer(s).
The customer here is responsible for the service. For overhead installations here, the customer is responsible for supplying and installing the main panel and disconnect (usually an integrated panel which includes the main breaker for disconnect here for residential installations,) the feed to the meter base, the meter base itself and the feed wires up to the masthead, leaving at least 3-4 feet (I forget the exact spec) minimum of wire hanging out for them to be able to make connection and form proper drip loops. You can pretty much pick whatever amperage main breaker you want as long as you have the appropriate size feed conductors from masthead-meter-panel disconnect.
Long, LONG ago, like 1950s, you were allowed as low as 60A if you wanted. 70A then became the minimum (and strictly speaking is still allowed for VERY small dwellings like a single-bedroom apartment) but any normal single-family dwelling or one side of a duplex like mine that is over 700 sq. ft. is required to have a minimum 100A service. Most typical homes here are now usually built with a 200A main breaker and appropriate input wiring from the start.