Yes, it is possible they used only one phase for the electricity in the house. Have seen that several times. Just think of triphasé as 3 separate monophasé lines of 230 Volts with respect to neutral, the blue wire. Indeed there is a phase shift between these three, resulting in the dangerous 400 Volts between them (L1- L2, L1- L3 and L2 -L3).
In a reply early this morning to Garry (subject: electric meter) I 've included some links. The included diagram of the triphasé installation (last link) is useful for reference. The only thing you have to think away in picture # 1 is the main switch - disjoncteur de branchement, DB TRI - and replace it with its monophasé brother. And draw one thick phase line to the three interrupteurs différentiel. And forget the 3-phase boiler (chauffe-eau) in picture # 2.
Three-phase systems have one inconvenience: if you were to have a puissance souscrite of 45 Amps in total, this power is distributed over the 3 phases. Each of them protected by the main switch (disjoncteur de branchement) at a trip current of 15 Amps. It is very easy to overload one of these phases, and then the main switch comes in, cutting off all of your electricity. Even worse: a 3 x 10 Amps connection (30 Amps total, 6kVa). Balancing the three phases is difficult (distributing the loads as evenly as possible over the three phases), especially in the smaller private home environment. With a 45 Amps (9kVa) monophasé connection it takes a lot of more loading to trigger the 45 A treshold in the disjoncteur de branchement. My advice to everybody - not being the proud owner of heavy 3-phase equipment - is to change these low-power 3-phase connections in a single-phase connection. You get a lot more of "headroom" before the overload protection in the main switch comes into action. It even allows you sometimes to go back - even EDF consultants are willing to point this out to customers - from e.g. 12 kVa to 9 kVa, or sometimes from 9kVa tot 6 kVa. Taking a look at the EDF tariffs today, this implies you have to pay less for your annual abonnement option base or option heures pleines/creuses. But perhaps better, for your own peace of mind and lighting conditions at night, to spend the few euro's extra to have plenty of headroom. By the way, speaking about tariff options: EDF is phasing out the option tempo. Existing connections go on, but suspended with the arrival of a new owner or new tenant (changement de nom). On almost all EDF (regional) websites this option has been removed. The same goes for its older sibling, the option EJP.
As long as you are connected to the EDF power grid, and pay your bills, the change from triphasé to monophasé is something following standard procedures. But not for free: nowadays they charge a standard E 135 and some cents for this intervention. Not a state company any more, private firm, less free service. Only when you want more power, several steps up the ladder, e.g. from 9 kVa to 18kVa, they are going to ask technical questions (is your installation safe and designed for the extra Amps) and perhaps send in the marines, sorry, the Consuel. One step higher is allowed without additional control.
For a new connection (raccordement), a major renovation or substantial power upgrade, the plot thickens. Then you are always in for a visit of CONSUEL to check what you 've done. You better do it, every inch, in conformity with the French NF C 15-100 regulations. Even the French, used to more than their fair share of bureaucracy, start trembling a bit, once they set foot in a maison pour controler tout ca. Otherwise no green light, no EDF connection.