I submitted 3 images several days ago and then several more yesterday. The images from yesterday have been reviewed, but the 3 from several days ago are still awaiting review. Is this normal?
As Tom Jones likes to say "It's not unusual."
Generally images are reviewed in the order they are received but every now and then the order can be altered within a portfolio. A possible scenario could include specific images being saved for a secondary review if there is a question of intellectual property or something else. I don't have specific information about your account in front of me, I'm just writing in general terms.
If the files aren't reviewed within a week of submitting them for review please feel free to submit a support ticket via the contributor portal and I'll be happy to investigate.
Please pardon the earlier pun. Sometimes I can't help myself.