You don't need to pack much on a 3 night cruise!
You are correct... there is no formal night.
On my 3 night cruise I had 2 pairs of dress slacks and 3 tops to mix and match. Hubby wore dress shirt and a tie to Tritons only and then wore a nice polo to the other two restauraunts.
If you like to wear dresses, bring one dress and one pair of slacks and 2 tops to wear. What ever you like. They try to take a photo of you in Triton's so if you dress nice it's fun.
Have a great time!!!