American Airlines ( AMR) says it will ratchet up the competition with low-fare carriers in Boston. Between March 2 and May 31, American will offer triple miles on flights from Boston to Los Angeles, San Francisco and San Diego, the carrier said Friday. (Non-stop San Diego service begins April 7.) Additionally, American has put all Boston flights on sale through May 20, with round-trip West Coast fares starting at $234. The promotion comes as low-fare carriers descend on Boston's Logan Airport. Most recently, Virgin America last month began nonstop flights from Boston to San Francisco and Los Angeles. "Our Boston bookings are among our strongest, and we feel we're far and away offering the best value in that market right now," says spokeswoman Abby Lunardini. JetBlue ( JBLU) and United ( UAUA) also fly nonstop between Boston and California. With a winter schedule heavy on Northeast to Florida travel, JetBlue is currently the airport's leading carrier. Its nonstop transcontinental markets include Long Beach, Calif., Oakland, San Diego and Seattle. It will add San Francisco on May 1 and Los Angeles on June 17. Still another transcontinental competitor is Alaska ( ALK), which flies nonstop from Boston to both Portland and Seattle. Additionally, Southwest ( LUV) said last month that it will begin service to Logan this fall, although it did not identify the routes it will fly. Logan has one of the country's most balanced airport service profiles, as five airlines each carry between 10% and 20% of its passengers, according to the Bureau of Transportation Statistics. Currently, under its winter schedule, JetBlue carries about 20% of the traffic.