Depends on your interests... all three have equally good career oppertunities.
If you do Oracle, and LAMP(linux-apache-mysql-php) later you can go in the database administration line, mainly in companies to maintain their internal/website databases, or even entire web servers.
If you do Java, you can work as a programmer, in a variety of applications, from mobile phone programming to web services to... well java is used in almost all fields, so its a good thing, but also a bad thing because you might need to specialize in one area in java (like j2me for mobiles) Someone correct me if I'm wrong, though.
If you do .NET its like java, you have a variety of applications in all fields. .NET is supposed to be a Windows-only framework, but organizations like Mono Project and MainSoft are working for using it in linux too. I dont know how successful they are, since I'm partial to Windows myself, but I see via google that they are quite successful in the area. I had the same question as you, but I chose to learn .NET technologies.
Correct me anyone, if I'm wrong anywhere, since I have the same question as Shloeb.