Adding ventilation and insulation with limited soffits
It looks like you have a ridge vent on the left already. The ridge caps look a little more "pronounced" than if there was no ridge ventilation. If there is no ridge vent then adding one would help a bunch.

Here is a section of a page from the 2012 IRC code stating the minimum requirements for venting. There are exceptions as well that should be in there too. It states 1 sq. ft. of vent for every 150 sq. ft. of attic. I can be reduced to 300 sq. ft. if you meet the exception. I think I read it right. But confirm it for yourself.



In the case of your one vent that was covered, it may have allowed snow to blow into your attic when there were high winds, then melt and cause drywall or plaster problems later.

If you feel you need to add more ventilation beyond the ridge vents, a continuous soffit vent will fit in that soffit space, it would be really labor intensive, but it would offer the most free air passage. compared to 3" rounds.