The carriers have all applied to the US Department of Transportation to operate the route.
SkyWest, which currently operates the route, said passengers would continue to benefit from access to the United Airlines network.
The airline, which is asking for a $2.35 million annual subsidy, said it would continue to offer 13 round trip flights a week from Los Angeles, using a 30-seat EMB 120 turboprop.
SeaPort outlined three separate offers: 29 weekly round trips to San Diego; 17 weekly round trips to San Diego and 12 to Phoenix; or 17 weekly round trips to San Diego and 12 to Burbank/Los Angeles.
The carrier said it would require annual subsidies ranging from $1.9 million to $2.7 million and would operate the service using a Cessna Caravan and a Pieper Chieftan.
Timothy Sieber, vice president for commercial, for SeaPort, said SeaPort’s proposals offered a 100% increase in the number of departures currently offered by SkyWest.
Sovereign said it would provide either 24 or 19 weekly round trips, with either 19-seat or 30-seat aircraft.
The carrier said it would require $1.85 million to $1.88 million in annual subsidies to operate the route.