I buy a B6 charger. I want to know that what is the ideal or safe current (A) to charge and discharge a 3s lipo.

Usually 1C, whatever be the number of cells or capacity.
I.e, 1A for 1000mah, 2.2A for 2200mah lipo.
As it reaches full charge, the B6 will automatically reduce the current.
As you will not normally use more than 60-70% of the capacity, 1C will give full charge in 35-45 minutes.

My B6 does not allow discharge at more than 1A.
There may hardly be the need to discharge a lipo via the charger. If you are not going to use the lipo for say 2 weeks or more, use the 'Storage' option. It will bring the voltage up or down (as reqd) to about 3.82-3.85v per cell (equivalent to 50% capacity). Lipos stored with full charge will usually puff up within a few weeks.