3.5 months post op Eyebrow transplant, only one brow is growing. Is this normal?

It may take 6 to 12 months to see complete results.  3.5 month is too early to tell if both your eyebrows will grow equally.

3.5 months is too early to tell what the final result will look like. Often, the transplanted hairs will grow unevenly.  I wouldn't sweat it at this point.

Usually, at 3-4 months, the grafts start coming out. So, if on one side, they have started coming, they may even on the other side within a month or so. However, if it does not happen, then it maybe that if the grafts being implanted on the left side got traumatised or dehydrated during the procedure which may lead to the unequal results.