The Universe Edition Group 3 Starter Sheet was used primarily to produce Starter Decks. This sheet contains five sets of twenty cards each, with each set of twenty cards being themed for one empire. When packaged into a starter deck all twenty of these cards would be added to a single starter deck. The five sets on this sheet were themed for the Clydon, P.O.T., Scorpead, Tufor, and Vektrean empires.
Universe Edtion booster packs also had some of these cards added. Slot 3 would be filled with ones random card from this sheet (thus calling this Group 3). Card rarity is based on how many times a card was duplicated on this sheet. The Common card appears four times, Uncommon cards appear three times each, Rare cards appear twice each, and Very Rare appear once each.
One card, the T/B5 Vektrean Asteroid Outpost, appears on both this sheet and the Group 2 sheet. Thus it can appear in Boosters in either slot 2 or 3 and can appear in starters of other empires as a terrain card.
Note that the depicted Print Sheets are just example layouts. I don't know what the actual card layout of these sheets were. In fact, the actual layout was likely more randomized to make it easier to pack boosters.