I imagine it will come down to where the Rams think certain players will fall, and then where they actually do fall.
But if things go as planned, the Rams will draft an Edge first, then a DT second. Might they double down on another DT? Sure. But not until the 4th round or later, most likely, unless the draft falls in a way that a DT they have marked is simply better than any other guy they are interested in.
PFF is awfully good at judging players and judging about where they'll go in the draft, so they are one of the 2 or 3 I pay any attention to regarding mocks. Of course, on draft day, trades have a tendency to shake things up.