Second Surgery to Remove Extra Skin Left from Blepharoplasty?

It is very common to have persistent swelling of the upper eyelid skin after surgery at 3 weeks, especially if fat was also removed. It may also be due to sagging brows causing more skin present before the surgery.
Therefore, I would wait at least 3-6 months to allow complete healing before any touch up removal of more skin. Then you can really determine if the eyelids are not symmetric.
And I would revaluate the position of the brows and if they are contributing to the extra skin fullness, you may consider an endoscopic brow lift. It is important not to remove too much skin of the upper eyelid, resulting in difficulty closing the eyelid or lagophthalmos.

Revision Surgery

Revision eyelid surgery should be done at least 6 months
after the initial surgery to allow time for healing and tissues to soften. Asymmetries, excess skin, fat rolls, fat malposition can all be improved. Other treatments may include laser or
chemical peels to tighten skin and improve texture

Assuming you didn't originally need a brow lift, I'd give this surgery some time before making a decision on its success. Three weeks isn't much time. You still likely have some swelling which will detract from your final result. Give it a couple months before approaching your surgeon about a touchup.

Since it has only been three weeks status post blepharoplasty, it is important to wait at least three months prior to undergoing revision eyelid surgery. This can be done easily under local anesthetic after full healing has taken effect, when all the swelling has subsided and the incisions have healed.

On occasion, there may be a small amount of extra skin on one or both upper eyelids following Blepharoplasty. In these cases, it is very easy to conservatively excise the excess skin with local anesthesia. However, three weeks post-op is too soon. The perceived excess skin may be related to post-surgical swelling. I would be inclined to wait at least three to six months before considering removing any apparent excess skin from the upper eyelids.