Hi Laura
You could use formula as below (built for month timescale)
QUARTERVALUE(Value) + POST(QUARTERVALUE(Value), IF MONTH(START()) <= 3 THEN 3 ELSE IF MONTH(START()) > 6 AND MONTH(START()) <= 9 THEN 3 ELSE -3)
You could also simplify IF formula if you build module (eg named Calendar) with month timescale and calculate in it the boolean property (Q1 or Q3)
in this case you get formula
QUARTERVALUE(Value) + POST(QUARTERVALUE(Value), IF Calendar.'Q1 or Q3' THEN 3 ELSE -3)
Hope this helps