Within the trio collection of Rheinberger are two or three that are not difficult (the g minor and the C major come immediately to mind). Flor Peeters' instructional organ manual has a section with pieces in trio style that are excellent introductory pieces. The organ literature is rich in pieces that are in three-parts, but not strictly imitative. Among the first trio pieces I ever learned was "Ich ruf zu dir" in Bach's Orgelbüchlein.

Georg Andreas Sorge's Trios are not difficult.

If you're looking to peel through some simple trios for sightreading practice, use http://imslp.org/