Ashlea,
sorry - a baptism with your twin sisters is even more special, you are really lucky to have them all together I think.
The size of your baby is so unpredictable, I had a growth study done three days before I went into labour and they told me my baby was at least 3kg but she was only 2.6kg when she was born, so could be better to wait and see.
Also, they can put on quite a bit of weight in that first month, my son was born at 3.4kg and by 4 weeks was 5.1kg so a gown might be a good idea so that you have flexibility with the length and it doesn't matter if it is a bit too big around the chest.

Goodluck with it and hope you have a lovely day.