It's hard to see from your pictures, but are those holes drilled through those pieces or is it grooves into the stone? If it's holes then those may be pieces of broken Bannerstones and if they are and were all found in the same spot I would go back and search that area very carefully to see if there are any more.
I would go back to the site they came from and look around for more pieces then. If you found all three of those on one site there is bound to be more and you might get lucky and find the other halves.
It is a spot you might have to keep going
back to. There should even be whole ones.
There are a lot of stories of people piecing
them together. Sometimes it could be the
next rain or next year or more, but it happens.