For Lecha Dodi yeshiva.org.il says:
Regarding bowing, R. Yakov Emdin brings that his father, the Chacham
Zvi, would bow three times when saying Bo’ee Kalah, to his left, then
his right and then in front. This is identical to the bowings
mentioned when we finish Shmoneh Esreh. The idea is because the
Shchina (Divine Presence) is, as if it were, before us (“I have set
Hashem always before me”, Tehilim 16, 8), so we bow to Her right (…
from His Right-hand (came the) fiery law”, Dvarim 33, 2) before Her
left, and then, as if it were, to Her.
A significant difference should be pointed out, that the bowings at
the end of Shmoneh Esreh are cited in the Talmud (Yoma 53b) and the
Shulchan Aruch (Or. Ch. 123, 1) and are universally accepted, as
opposed to those at the end of Lecha Dodi, which is not cited by most
of the poskim and is a relatively recent custom of only some
communities.