The 50mm 1.4 and the 5D3 are a great pair. My wife keeps the 50mm 1.4 on her 5D3 by default and only changes it for special purposes (mainly if she wants telephoto with the 70-200mm). For family gatherings, etc., she keeps the 50mm on and gets great results.
As others have said, the 50mm 1.8 is actually quite good, too, considering it only costs around $110. But, we still prefer the 1.4. If you can afford the 1.4, then I'd say go for it, as you'll probably want it eventually.