You have access to all 3 Emirates first class lounges.
There's a separate first class lounge in concourses A, B and C. The offerings are pretty much the same but some features are different. They are all very good lounges (showers excepted, perhaps).
Concourse A is probably the best. It's vast, seems the most modern and has the best showers by far!
Concourse B is a similar size to A but seems older and has horrible showers.
Concourse C is much smaller than A and B but is kind of nice in that it's got it all in a more compact area. But it has (apparently) the horrible showers again.
When it's getting closer to departure, you really should go to the lounge closest to your gate.
You can reach all 3 lounges if you have time but there's quite a distance between them. Between B and C for example is about a 20 minute walk. There are bus links between them all and a train link between A and B.