In Grade Three you are introduced to 'compound time': this is where, instead of beats subdividing into pairs (e.g. of 2 quavers within a whole crotchet), they each split into groups of three shorter notes (typically, quavers within a dotted crotchet). You therefore need start to using different types of time signatures that count in quavers ~ so they have an 8 as the bottom number of the time signature, while the number above it will be a multiple of 3. (Rather appropriate that this happens at Grade 3; provided you know your basic times-tables, so you can make sense of such time-signatures as 6/8 and 9/8!)