The word "sol" and "plancher" are almost synonym but "plancher" is often used for a house's floor while "sol" is used for everything. The word "terre" are differents senses: -"Terre" with uppercase is Earth, the planet. -"terre" is also a substance, like mud. -"terre" is also the floor, but we use that word just when the floor is is earth

For me sol and plancher can be synonyms, but plancher is man made whereas sol can be made of dirt or man made. Placher can be made of anything (but then I am Québécoise!), you need to specify: "Plancher de bois franc" "plancher en ciment", etc.

Le sol: Soil, le plancher; floor , la terre:The earth

There is also étage, if you´re looking for the sense of Ground floor, 1st floor etc :) in fact you have to specify the usage as there are so many words you could use.