How can I get an element of a sequence?

seq = Sequence[a, b, c, d]

Sequence[a, b, c, d]

You can use a composition of Part ([[...]]) and List as suggested by J.M.'sennui:

{seq}[[3]]

c

(Part[#, 3] & @* List)[seq]

c

Alternatively, you can use Slot[3] (that is, #3):

#3 & @ seq

c

Slot[3] &[seq]

c