These three words are terms which belong to the Geology field.
The first one is a sheetlike igneous body that tends to be oriented vertically or steeply inclined to the bedding of preexisting intruded rocks.
A sill is a flat sheetlike intrusion. As molten magma, it pushed between older layers of rock. The older rock could be beds of volcanic lava, tuff, metamorphic rock or sedimentary rock. Alike the one explained above, sill does not cut across preexisting rocks.
Laccoliths can be contrasted with sills. It is any sort of igneous intrusion that has split apart two strata, giving rise to a domelike structure. Laccoliths tend to be smaller than a stock, and usually is less than 10 miles in diameter.
