How could I extend a baby gate at the bottom?
Measure your baby. The 3" gap is based on (a) minimum baby size across all babies expected to use the units, plus (b) a generous quantity of CYA on the manufacturer's part. Thus, 4" is likely just fine, and you can verify this by figuring out the minimum gap that your actual baby can squirm through, plus enough extra to satisfy you and your wife (reasonable and sane people) rather than lawyers who smell blood (and/or money).