The total number they were using must be divisible by $3$.

Ben's counters must initially have been divisible by $3$, Jack's by $4$ and Emma's by $5$.

It might help to work out the maximum each could have started with - e.g. Emma could not have started with $25$ counters. Can you work out why?

How many counters could each of them have started with?

Try some possible numbers.