Can I send my accepted research paper to IEEE conferences?
I think 3 published papers in reputable journals (though you do not
say which precisely) should more than enough to get you in a PhD
program. Dome people are happy to get as much with their PhD work.
But that may depend on the field.

However, publishing twice the same result, even with some cosmetic
changes, is a good way not to get accepted for a PhD program, or for
an academic position.

People publish far too much, and waste each other's time. We have to
strive for quality, not quantity.

Now, you did not specify you field, and thing may somewhat differ
according to scientific field.

In some fields, conferences are considered publications, sometimes
harder on selection than (some) journals. In other fields, it is more
acceptable to republish in conferences as conference papers play a
minor role and are more intended as topics for meeting with
colleagues, and giving introduction to your work.

The former will have more importance for your publication list, but is more demanding on originality.

The right thing to do, that will avoid any problem, bcause they will
be dealt with by the program committee of the conference, is to be
extremely clear as to what is already published, to make it visible
and to have in the references your previous relevant publications (as
well as those of other people), whether already accepted or only
submitted. Then the program committee will let you know whether your
submission is appropriate or not.

But do not abuse relying on the program committee, they have considerable work to do.

And always be very careful when using twice the same research results.