Meanwhile across the North Seathe three Germanic tribeswhich were destined to form the main element in the English race weremultiplying and unconsciously preparing to swarm to their new home.
这句如果非要断句（加逗号）应该咋加呢

这个话题太大，交叉的专业太多，跨越的历史太长，涉及到的范围太广。根本不可能七嘴八舌说的清楚(三言两语就更难了)

Meanwhile, across the North Sea, the three Germanic tribes, which were destined to form the main element in the English race, were multiplying and unconsciously preparing to swarm to their new home.
给你加了4个逗号

英语是古代传下来的

顶lz