The tank WILL overflow.
60*30*40= 72000cm^3
The max volume the fish tank can hold: 72000cm^3
amount of L * 1000= cm^3
70L * 1000= 70000cm^3
There is already 70000cm^3 of water in the tank
70000cm^3 + 3000cm^3 = 73000cm^3
The tank can only hold 72000cm^3 and the water and rocks are 73000cm^3, so the tank will overflow.