Or HaChaim on Exodus 3:16/אור החיים על שמות ג׳:ט״ז

1 לך ואספת את זקני ישראל, "Go and gather the elders of lsrael,etc." The reason that G'd repeats here again what Moses is to say maybe that when Moses would arrive in Egypt he was to tell the entire people that he had come as messenger from G'd. After that he would assemble the elders and tell them the nature of his mission, namely that G'd had remembered His people and was about to redeem them.

2The reason that Moses was to announce the fact that he came as G'd's messenger to the whole people first, was to make them assemble around him. Who would bother to gather around him unless he had first made an announcement that would make the Israelites take notice of him? In His wisdom G'd had instructed Moses what to say to the Israelites and what to say to the assembly of elders. He was to address the elders separately out of respect for them as we know from Vayikra Rabbah 11,8 that in the future G'd will allocate a special place where the elders will be seated.

3פקוד פקדתי, "I have surely remembered, etc." G'd speaks of two separate remembrances, 1) the final stage of the redemption, 2) the end of their suffering. When G'd says: אתכם He referred to the final stage of the redemption, when He said: ואת העשוי לכם He referred to the torture the Israelites were experiencing while performing slave labour. G'd also hinted to the Israelites that He would punish the Egyptians for what they had done to the Israelites already.