My opinions only, no facts here:
I have not posted on this particular case before, but I can be found elsewhere on websleuths.
If an infant is missing, very difficult to find. If one small adult is missing, somewhat difficult to find, but you might get lucky. If THREE people are missing- much easier to find. It is all about the logistics. If one person kidnaps three people, they must carry or escort them from point A to point B and later, from point C to point D.
Sink three people in a body of water, and evidence WILL surface, even if you use concrete overshoes. Toss three people at or near the surface anywhere on public land in the US and after a few years someone will stumble on them. Bury three people six feet deep and they will not be found for a long time, if at all. But a six foot hole is a big job with a hand shovel. Crimes like this (unless committed by organized crime) require "special access". Both to equipment and to private property.
I would surmise that a construction site, junk yard, or farm would be reasonable sites to explore.