I think something changed when Ben did the most recent forum upgrade. When the site came back up, I had a problem posting a .png image. Did not check the file size, but I don't think it was any bigger than other photos I've posted before. I just saved it as a .jpg and that worked. I recall that Guy also mentioned he had a problem at that same time. So the size limit may have changed.
But 3MB is pretty big to be posting here. On my iPhone, unless I re-size the photos they range from 2MB to almost 5MB each and 3264x2448 pixels. That is way too big to post in the forums, I usually re-size them to 800 pixels wide unless it's something where a lot of detail is needed - such as a map.