calf (baby antelope)
Ibex
Buffalo is not really an antelope, but it is the closest relative to the antelope where the name ends with an o.
Ted
gnu
Dik-dik, it is small antelope that lives in the bush of East Africa
dik dik
Ibex?
Eland is an antelope
Oribi is a small African antelope
eland
kudu
Perhaps.............gnu?
A three letter African antelope is the "gnu", which ends in a "u" not a "you". The more common name for the gnu is the wildebeeste, but that won't fit into your crossword puzzle.