Medical residency applicants do get confused as to whether they should take it before applying for residency or after they have begun the program. However, there is no absolute answer.
A Step 3 USMLE can strengthen your application no doubt but not as much as you might hope. As of 2014, the NRMP program director survey stated that the last factor considered when offering interviews was the Step 3 USMLE score. Candidates who have recently graduated from the medical school may also want to reject the thought of writing a USMLE step 3 until they have acquired enough training and medical knowledge for the exam.
However, this doesn't imply that the Step 3 USMLE is of no use. It is suitable for mature candidates who wish to step up the values in their application. It is also of great benefit to International Medical Graduates.
As an IMG, there is a need for you to pass to your USMLE. The program directors might be interested in it for a lot of reasons:
They are confident that you will scale through your residency test
With a USMLE Step 3, you do not have to worry about previous USMLE attempts on other steps. Many failed attempts is an indication of what your performance will be in other tests.
You won't have to go through the stress of studying for the test while doing your residency.
Many states require that you pass your Step 3 USMLE before completing your third year of residency.
To stand a chance of getting an H1-B Visa sponsorship, you will need the USMLE step 3.
By passing the USMLE Step 3, you are instilling confidence in the program you are applying for. Most importantly, if you wish to get your step 3 done, be sure that you have made adequate preparation. Failing a USMLE Step 3 is worse than not getting it done at all.