change resolution of jpeg

Do you want to increase the pixel size of a photo or just change the embedded "Dots per inch" tag, so it can be printed at a larger size?

If you need to add more pixels to the photo without blurring the edges, you need special software that can interpolate more pixels smartly. I am using the free ImageEnlarger - you can upload the images to http://www.imageenlarger.com/

There you can try several different algorithms to enlarge the photo and pick the one with the most pleasing result. It will probably not be perfect, because artifacts will also be enlarged, so you may need to retouch the result a bit.

I'm told it has to be at least 3 MB

The editors of the magazine are not very professional then. The file size in MB does not say much about the print quality of a JPEG photo. Important are the pixel size and the JPEG quality (the amount of compression)

How did you download the photo from your iPhone? Did you transfer the photo by my Photo Stream to the Photos.app on your Mac Book Pro? Or where do you have the photo now?

If your photo is currently in your Photos Library on your Mac, use "File > Export > Export ... photos" to save a high quality version of your photo to the Desktop.

With these settings you should get the highest possible File size: