Yes you can – by taking a 3D scan of your body you get a perfect model of it. You can then design the armour around that model and when it’s printed it will fit you perfectly.
Recently I saw an example on the internet of someone who’d used 3D printing to make a complete set of Batman armour for him to wear (presumably for when he went out in Gotham City!). That was a pretty cool project.
3D printing is very good at making things to fit you perfectly, I know soon there will be a development that uses facial scans and 3D printing so that if you wear glasses (or sunglasses) you can have them made to fit perfectly to your face.