"You are nice!"
What Troll1995 said, except καλή is the feminine form, whereas the other two are masculine.
Ωραίος, καλός, όμορφος.
Ωραία, καλή, όμορφη.
Ωραίο, καλό, όμορφο.
Grammatically, yes, but due to social conventions, people may not use every grammatically correct structure possible.
People would mostly say something like "Είσαι πολύ όμορφο αγόρι/κορίτσι" when addressing children.
There's also this sort of fixed phrase, "ωραίο παιδί" when referring to male teens and young male adults. Hope this helps a bit!