Carpels are the basic units of the gynoecium and may be free (distinct) or fused (connate). The term pistil is used in a similar manner to carpel – in some situations the terms are equivalent in meaning but not in others. For example, a flower represented by G 1 has a single carpel or a single pistil. A flower represented by G (3) represents three fused carpels but is only a single pistil (compound).
One flower with
In a floral formula this would be expressed G3_
One flower with
In a floral formula this would be expressed G(3)_
If the above is confusing the simplest path is just to use the term ‘carpel’ and avoid the potential ambiguity of using ‘pistil’.