When the upper eyelids cover the iris of the eye by 3 mm without any vision problem, would you recommend ptosis surgery?

Cosmetic ptosis surgery, and any other cosmetic surgery is a "want" more so than a "need". So only you can decide if a feature is bothering you enough to nudge you to have a procedure done. Having said that cosmetic eyelid ptosis can create a subtle improvement that is very pleasing to the eye. It is impossible to tell how your ptosis can progress over the next few years. Some will get worse, while others will stay the same. 

We can perform an in office pre-surgical simulation that will help you decide whether or not you would like to go forward with surgery. Below is a link to my cosmetic ptosis gallery so you can get a good idea of how this can improve the eyes. 

Based on your description you can benefit cosmetically from ptosis surgery such as the examples in the link below.  It's hard to know if your ptosis will get worse over time or not

it really should be what you want.  If you are fine with your look, why spend the money on a procedure you don't really want?  If you want a more open look to where you lid is closer to the edge of your iris, then you could benefit from ptosis surgery but perfection is really really hard to accomplish and having one lid a little lower is a real possibility as well as over-correction where the whites of your eyes show.

There are many causes for ptosis and it would be necessary to know the cause to make a good recommendation. Cosmetic Blepharoplasty with Ptosis repair can make a cosmetic improvement even without a visual field loss.  Depending on the cause the ptosis may get worse over time.  See our thoughts in the link below.