Insulating an unused three story chimney

[mozymat]

I have a 3 story 1970s house with two flues in the chimney. The fireplaces are on the basement and main floor level. The chimney is located on the north exterior wall of the house. I removed the firebox and part of the flue from the basement level and the main level currently has a non-working gas insert (Gas line has been removed from that side of the house) . I want to put electric inserts in both openings and seal up/insulate the chimney which is a giant heat suck for the house. Rooms in this area are always much colder then the rest of the house with the basement rarely getting above 50 degrees in the winter. There was insulation in firebox in the basement opening which seems to not have helped at all.

What would be the best way to go about fixing the cold? I am attaching photos of the openings on the basement and main level and pictures looking up from the basement opening to the flue that was removing from there and the existing flue for the main level.