No, don't move the pattern out 3" from the center fold. This will cause you a ton of problems.
1) You won't be adding 3" to the pattern by doing this. If you make this adjustment to just the front or back, you're actually adding 6". If you do it to both, you're adding 12".
2) Not only would you be adding to the waist, but you'd also be adding to the hip, chest, and neck circumference. The entire garment becomes a sack.
You need to distribute that 3" evenly among all seamlines in the garment, taking into account the pieces that are being joined. You also need to locate the waistline on the pattern.
For instance, if your garment just has side seams and the front and back are BOTH on a fold, this means you have four join edges. Right front, left front, right back, and left back. 3" divided by 4 is 3/4", or .75". This means you would add 3/4" each to your front AND back pattern piece at the side seam, ONLY at the waistline. You then gradually taper the increase back into the original seamline both above and below the waist.
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