A 3S is less than ideal for a 1/8 electric buggy. Generally requires a high kv motor which will run hot, have somewhat limited gearing options, and not so great run times.

Better to have at least a 4S, which is the norm for the class. Depending on track rules where you plan to run if any, some go higher than 4S with lower kv motors.

On the 3S it would be wise to check the cell voltages, make sure they are reasonably close and at least above 3V each. Even better if they are above 3.7V per, should get a little stronger after a couple cycles.