Could a manuscript be under review even if the AE was not assigned?
I tend to disagree with @Allure.  I have been an EIC, and I'm an associate editor of several IEEE and ACM journals.  The manuscript was submitted only 3 weeks ago.  It is possible that the EIC just hasn't gotten to it yet, or is having trouble finding the right associate editor to handle it.  I think the system will say "under review" as long as it has been submitted and not yet accepted or rejected.  

It is possible that the EIC is trying to decide whether to assign the paper or reject it immediately, but I think that's not the conclusion I'd jump to.  

OP should simply be patient.