What size wire do I need for a 600amp /3 phase service?

Assuming you are in the US, you could use one set of 1250 kCM copper or two sets of 300 kCM copper or three sets of #3/0 copper...
The above taken from NEC Table 310.15(B)(16).
Be aware you may need to make adjustments if you are traveling a large distance or if you are in a high ambient temperature environment or when using multiple sets of cables you install them underground.

And you thought this was a simple question!