Depends on whether or not you feel the cold
I dont and I did a March Livaboard wearing a 5mm one piece and was comfortable. Water temp was 22 degrees and air temp was between 18 and 22 degrees although it felt colder in the wind.
3mm is possibly a bit thin for that time of year, the Shadow shorty over your existing 3mm is a good compromise which I have seen many people use successfully.
Let us know how you get on
Regards
David