My understanding is you pick a single server and your soul pack is applied to that server. If you wish to swap servers you need to buy another soul pack and at least 1 more spark.
I don’t believe your initial starting souls give 3 per server.
To begin you must select which soul you want to use. Each account starts with three souls and the option to add more by purchasing Soul Packs. Most souls will be blank slates that are "Once Born" meaning they have never lived a life before this one. However some souls will randomly be assigned past lives with any combination of skills and affinities.