Since they were of three different sizes, I decided to base them on different-sized round lip bases.
Then I got the idea that I should make an area-terrain piece with three round slots for them that would allow me to remove the individual bases to facilitate placement of figures onto the terrain. The base is basically a piece of of mounting board withe the appropirate sized holes cut into it glued to another, slightly larger piece of mounting board. The edge is given a bevelled effect by masking tape, and the whole thing is textured with some cork pieces and a mix of sand and gravel, then sprayed and painted and finally flocked.
Then I thought: since I am going to make a base, I might as well think of other things I can place onto it to represent other types of terrain!
So after some googling, I bought a pack of these from Fantascape:
I painted them to match the rocks on the base, so that when placed on it the whole will look likethe site of some ruined building.
I'm now making a set of water effect inserts to allow me to use this as a marsh; and once I get the bits I plan to make an undead pit of some sort. Stay tuned.