I would like make an animation in GeoServer using three shapefile. I looked this page https://docs.geoserver.org/stable/en/user/services/wms/time.html#specifying-a-list-of-times but I didn't understand how I can create a "time specification". How can I do this?
Time series in GeoServer tend to be collections of rasters or single vector files.
So you need a single shapefile with a date column in it, the animation will then allow you to show the various features in the layer that match the time settings.