QUICK HITS.

Third's a charm: TriStar Airlines recently added a third daily nonstop round-trip flight between Los Angeles and San Francisco international airports. Flights leave Los Angeles at 7 and 11:40 a.m. and 4 p.m.; flights depart San Francisco at 9 a.m. and 1 and 6 p.m.

Starting June 12, TriStar will also fly three round trips a week direct to Aspen from Los Angeles for 14 weeks, offering one round-trip flight every Wednesday, Saturday and Sunday. Fares start at $378 round trip, with a special introductory, non-refundable round-trip fare of $298, which must be purchased by May 10. Summer flights will depart from Los Angeles to Aspen at 11 a.m.; flights to LAX will depart Aspen at 2:45 p.m. Information and reservations: [phone].

Desert safety: The Clark County (Nev.) Commission has ordered a feasibility study on a proposal to install emergency telephone call boxes along Interstate 15, Interstate 95, Highway 160 and Highway 93, all main thoroughfares leading to Las Vegas. Thirty-two percent of Las Vegas visitors are from Southern California, and 66 percent of them arrive by car via Interstate 5, say Nevada officials.

Flying machine: Visitors to San Diego can go air-touring in vintage open-cockpit biplanes with Barnstorming Adventures. Prices begin at $98 for two on a 20-minute flight. Passengers get to wear antique helmets and goggles. The pilots dress up in antique flying suits. All pilots are experienced and, quoting a 1926 ad, ``... are sober, don't chew or spit, and have never cracked up.'' Information: [phone].

Leave a message: Rent an Avis car in Britain, France, Germany, Italy, Portugal or Spain and the company will act as a clearinghouse for messages during the rental period. Renters must call the Avis Know Before You Go line (at [phone]) to get special toll-free message-center numbers before they leave for Europe. Information: [phone].