"Sport" does have a singular form. Football is a sport. Baseball is a sport.
"Series" also has a singular form: it just happens to end in an "s." A new series is beginning.
The plural for "series" is the same word. Several new series [multiple sequences, such as the whole fall lineup] are beginning. That's not unique: "species," for example, is both singular and plural.
That's the problem with the sentence. If you write "Drei Serien" in German it can mean "three series" of a car model" for example or "three different shows". Same issue with "camp" which can translate into Lager (which in German can mean a warehouse or a stock. I wish they could be more precise with the sentences. You could easily make sentences more logical by adding "three new toy series" or "drei neue Spielzeugserien" if you expect a specific word to be used.