From where are you coming, on your way to Scandinavia? How are you getting there? Where do you intend to go, once you’re finished with your Scandinavia leg?
If you’re starting by flying to Copenhagen (as we did this past September, and intend to wind up in, say, Estonia or Latvia or Lithuania or Poland, then go from west to east. If you intend to backpack thru Scandinavia, then swing south (Germany, Netherlands, Belgium, France, etc.), then starting in Finland and going from east to west makes sense. Are Finland, Sweden, Norway, and Denmark (but not Iceland?) all intended?
This past September, we visited parts of Scandinavia - Denmark and Norway - but weren’t backpacking. We rented an electric VW in Norway, driving thru fjord country and high mountain terrain (where camping huts for backpacking were abundant), but also used relatively cheap Norwegian Air for flying long distances within Norway, and from Copenhagen to Trondheim, and Oslo back to Copenhagen. Trains, at least in Norway, are expensive but effective. Maybe buses would be the way to go., if cost is a concern. In Denmark, we used trains for trips beyond Copenhagen, to Helsingør and to Roskilde, then buses within town. Bicycles are are big in Denmark, Netherlands, and Belgium, if bikepacking for part of your time is something you might entertain.
Are the British Isles part of your plans? To reach them, a ferry might the way, unless you flew, or took the Eurostar train thru the Chunnel. Even with with 3 months, you’ve got a lot of ground to cover, so if trains are part of the equation, some kind of train passes might be warranted.