What requirements do you have? What external libraries or functionality do you want to use?
Which OS are you running on, WinXP?
What are you familiar with, how strong are you in these three languages?
Personally I would look at going to Scala, using LIFT, as it runs on the JVM, but, if you don't have anything done yet, then you may want to start with PHP, so you can get something done quickly, and at least have an alpha version up, so you can show potential investors.
Once you know what features you want then you can redo it in a cleaner fashion in .NET or Java, depending on your answers to the questions above.
By going to .NET, if you worry about speed or scalability in parts then you could write those parts in F# and get some improvement, for example, but Scala will compile both CLR and java bytecodes so it can work on either platform, so scalability is less of a concern, if you can design in multi-paradigmic systems (I think that may be a new word, may need to be spelled 'paradimic').
If PHP meets your needs and you can scale it well, then you can continue to use it as your front-end and just look at making changes to the backend.
My point is that all three are used in large websites, so any of them will work well, the correct answer will be based on your answers above.
For example, if you need Comet, so you can have socket connections that stay open for a long time, then PHP will have a problem, then you are using .NET MVC if .NET can do it at all, otherwise you are looking at some other language.