Complex sentence: When Kernel joins them the three form the Kah-Gash (a weapon that could destroy a universe, when Kah-Gash is formed neither Grubbs, Kernel or Bec have control over their magical abilities) but they use it to reverse time.
Split into simpler sentences: When Kernel joins them the three form the Kah-Gash (a weapon that could destroy a universe). When the Kah-Gash is formed neither Grubbs, Kernel or Bec have control over their magical abilities, but they use it to reverse time.