Complex sentence: One of the most noticeable traits is that a monocot's flower is trimerous, with the flower parts in threes or in multiples of three--having three, six, or nine petals.
Split into simpler sentences: One of the most noticeable traits is that a monocot's flower is trimerous, with the flower parts in threes or in multiples of three. That is to say, a monocotyledon's flower typically has three, six, or nine petals.