ABC and Lorimar TV have reached an agreement renewing three series from sitcom factory Miller-Boyett Prods. — including a two-year extension on the 6 -year-old comedy “Full House.”The web also has agreed to pickups for next season on both lead-off series from its Friday “TGIF” comedy lineup, Miller-Boyett’s “Family Matters” and “Step by Step,” currently in their fourth and second years, respectively. Terms weren’t disclosed, but ABC confirmed that the shows had been renewed. Securing “Full House” for two more seasons provides a major boost for ABC, since the show has become an important anchor leading off the web’s Tuesday lineup since moving there in August 1991, providing a perfect time period for introducing new shows between “House” and “Roseanne.” ABC launched Disney’s “Home Improvement,” now a stand-alone hit, in that hammocked slot last season and this year has been airing “Hangin’ With Mr. Cooper,” which entertainment prez Ted Harbert recently said has “an excellent chance” of returning next fall, either Tuesday or elsewhere. With “Full House” extended two more years, “Home Improvement” secured into 1996 and a recently confirmed two-year renewal on “Coach” (Daily Variety, March 26), ABC has established the backbone of its comedy lineup through the 1994-95 season. The one major variable remains “Roseanne,” renewed last year through the coming ’93-94 term but with indications that its stars might want to pack it in at that time. ABC sacrificed some strength Friday with the move of “Full House,” but “Family Matters” and “Step by Step” have nevertheless consistently won the 8 o’clock hour since the shift, holding off older-skewing CBS sitcom competition. A fourth Miller-Boyett sitcom, “Getting By,” has performed respectably since joining the alphabet web’s “TGIF” lineup at 9 p.m. in early March. Strong ratings “Full House” averaged a 17.0 rating, 27 share last season (from mid-September through mid-April) in its Tuesday slot and remains on a similar pace this year with a 16.1/25, despite growth for CBS’ “Rescue 911″ and new Tuesday competition from Fox Broadcasting Co. Demographically, it’s the top-rated prime time series among children and ranks 10th in terms of the key sales demo of women 18-49. Producing four concurrent sitcoms, exec producers Thomas L. Miller and Robert L. Boyett rank among the most prolific tandems on network TV, along with Tony Thomas and Paul Junger Witt (involved in five current series) and Linda Bloodworth-Thomason and Harry Thomason, who produce a trio of CBS comedies. The deal also continues a strong relationship between Lorimar and ABC, now airing eight shows supplied by the Time Warner unit.