If your deca is dosed at 250mg/mL, then 3 vials will give you
250mg/mL x 10mL per vial x 3 vials = 7,500mg of deca.
7,500/mg / 16 weeks = 625mg/week
So yes - you will have more then enough to run 400mg/week for 16 weeks.
I like the dosages @Stevesmi recommended
Remember to include the proper support supplements with your cycle:
1. An AI to keep estrogen in the low-normal range. Aromasin at 10mg EOD is a good dosage.
2. An anti-prolactin drug to keep prolactin in the low-normal range. Cabergoline at 0.25mg EOD is a good dosage.
The support supplements are very important because high estrogen and prolactin levels can lead to side effects like gyno and water retention.