Complex sentence: After just three years of traffic the first generation ships were replaced by three new ships of similar but larger construction: the "Svea Corona" and her sisters MS "Wellamo" and MS "Bore Star".
Split into simpler sentences: After just three years, three new ships of similar but larger construction replaced the first generation ships. The new ships were were the "Svea Corona" and her sisters MS "Wellamo" and MS "Bore Star".