In order to fulfill the mitzva, all four species must be taken. If any one of them is missing, the mitzva cannot be fulfilled (Menaḥot 27a). Le-khatḥila they should be taken together, the lulav bundled with the hadasim and aravot. However, be-di’avad, if someone took them serially, he has fulfilled the obligation (SA 651:12; see 5:2 below).
This halakha teaches us something profound. Our Sages say that just as among the four species there are two that produce fruit (lulav and etrog) and two that do not (hadas and arava), so too, in Israel there are Torah scholars and people of action. Just as the presence of all four species is necessary to fulfill the mitzva, so too the presence of both scholars and doers is necessary for the nation to thrive. Scholars cannot survive without doers, who help support them; and doers cannot survive without scholars, who enrich their lives with spiritual content and help connect them to the next world (based on Menaḥot 27a; Ḥullin 92a; Tanḥuma Emor; R. Yitzḥak ibn Gi’at, Hilkhot Lulav).
A more elaborate midrash explains that the four species represent four types of people. The etrog, which both tastes and smells good, corresponds to Jews who are full of both Torah and good deeds. The lulav (date palm), whose fruits taste good, but which has no smell, corresponds to Torah scholars who are full of Torah but do not perform many good deeds. The hadas, which smells good but has no taste, corresponds to people who perform good deeds but are not Torah scholars. The arava, which has neither taste nor smell, represents simple Jews who do not have much Torah or many good deeds. At first glance, we might think that their lives are not worth that much, and they will be unable to reach the next world. But God says: “Bundle them all together, and they will atone for one another.” When this is done, God is exalted and the supernal chambers are built up, as we read (Amos 9:6): “He builds His chambers in heaven when His bundle is established on earth” (Vayikra Rabba 30:12).
Our Sages further suggest that the four species correspond to the founders of the Jewish people: the three patriarchs and Yosef, or the four matriarchs. They also suggest (based on wordplay) that the species hint at the Sanhedrin and Torah scholars (Vayikra Rabba 30:9-11).