I add formula supplement to breastfeeding my 3 weeks old. Does formula neutralize breastmilk benefits? I have heard so

Dilutes some. If you look at the antibody protection afforded in natural milk, there is none in formula. Supplementation does reduce the antibody intake/day in proportion to how much supplement is taken. For most, it has no significant effect.
No problem. After a baby has learned breastfeeding, it is ok to supplement breastfeeding with formula. Usually, supplementation is not needed unless the baby is very hungry after breastfeeding and can't wait until the next breastfeeding, or unless the baby isn't gaining weight properly. A baby on formula grows up fine, just as a baby on breast milk does. The formula doesn't "neutralize" breast milk benefits.