You'll hear pros and cons about keeping 3 fish in a 10 gallon, but many people do it and have success. I just added a purple firefish goby to go with my 2 clowns last week .... so far so good. So yes, IMO you can do it.
Don't add all 3 at once though .... put the clowns in first and wait a couple of months to let your tank mature more.