There are two bonus packs included with Lemmings Plus 3.

LP3-R.set contains the two rejected levels, "Desperation" and "Into Eternity". Desperation was rejected due to unfixable backroutes, and Into Eternity was rejected due to being too hard to pull off.

LP3-E.set contains easier versions of 21 of the levels that could be fun to play around in, or you could possibly find something by experimenting in. This includes the four training levels, "Lane Of Lament", "Catastrophe", and a few of the harder Kozuka, Wakizashi and Zanmato levels.

These are not part of the actual pack however.