The Arba Minim – Lulav, Esrog, three Hadasim and two Aravos
together constitute one mitzvah and must be held together in
order to perform the mitzvah correctly. If one of the four parts is
missing (and he cannot acquire it), he may not say a brocho over
the others, but should nevertheless put the other parts together
and perform the mitzvah without reciting the brocho, so that the
performance of the mitzvah not be forgotten for the future.
However, in doing so, he should not have any Kavanah that he is
somehow performing the mitzvah this way, as this would create a
possible problem of Bal Tigra (reducing mitzvos). (MB and Biur
Halacha 651:12)

Adopted from the shiurim of Ha Rav Leible Sternberg