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.

Yes hair growth after a hair transplant may start patchy and uneven. There maybe different rates of growth and healingi would give it another 2-3 months before worrying. Alternatively you could try applying minoxidil in the hope of accelerating the growth 

If two people were placing the eyebrow hairs at the same time and one was less skilled than the other, then you might see a difference between eyebrows related to the people who placed the grafts. Wait another 2-3 months before drawing any conclusions as the second eyebrow may yet grow. 

Totally normal!  Your grafts are just starting to grow in so you should expect a sparse and patchy appearance at first.  3-4 months is the right time frame for this to start and the other brow does usually catch up by 9-12 months. One word of advice, though; let your surgeon know.  If your surgeon is as interested in quality control as I am, they will have noted if one of their staff worked more on one side than the other.  That way if it DOESN'T even out, you will have already started the investigation into the cause.Good Luck!