what charge current I should use?

[Jabe]

If you charge them in series (total 6s) then 1C should be used (5A).
If you charge them in parallel (total 3s) they you can use 10A (1C as 5000mah+5000mah=10Ah).
1C= 1*XXXmah (1000mah=1Ah), so with 5000mah battery 1C is 1*5000mah=5A etc.

Charge rate "C" never increases, either you put batterys in parallel or in series, only overall capacity and voltage changes.