“Zayd and Amr arrived” uses the conjunction wa, which joins the two without itself specifying order. “Zayd came and then Amr immediately followed” illustrates the succession associated with fa. “Zayd came; later Amr came” illustrates the interval expressed by thumma.
Translating all three simply as “and” or “then” can erase distinctions relevant to legal wording. An interpreter must examine whether the particle indicates coordination, immediate succession, or delayed sequence in context.