yada and veya means or and both of them are the same
yoksa means if not or otherwise " I haven't seen it used as or " . when turkish people wants to say "or " they use yada veya " if it's in a sentence"
if it's a question they repeat the question mark " mi" .
for example :
- iyi mi kötü mü ?!
( is it good or bad )
- ne alacaksın ? uçak mı gemi mi ?!
(what are you going to take ? plane or ship ?! )
for example :
sport yapmam lazım, yoksa şişman olacağım .
( I have to do sports , otherwise\ if not I'll become fat )
I hope you understand it !! good luck