Timothy,
The problem is that your expression refers to the prototype objects used to generate the entities, instead of the generated entities themselves. It looks like you want the service time distribution to depend on the type of entity. The easiest way to do this is to define an attribute 'type' for each of the three prototype entities (ClientClasse1, ClientClasse2, and ClientClasse3) and set its value to 1, 2, or 3, respectively.
With this change, the correct expression for ServiceTime would be:
'this.obj.type == 1 ? [NormalDistributionCiasseAutoCaddi].Value : this.obj.type == 2 ? [NormalDistributionCiasseAutoPanier].Value : [NormalDistributionCiasseAutoMain].Value'
Note that this.obj returns the entity being processed by the EntityProcessor.
Harry