I distinctly remember a UA CSR posting here on FlyerTalk that "SFO" is the 3-letter code for SFO, OAK, SJC. However, the problem with SFO is that UA has a hub at SFO and therefore has most options that match your requested itinerary out of SFO and thus will display only SFO. Still, sometimes I'll get OAK and SJC options by using SFO, depending on the itinerary.

Similarly, LAX is the code for LAX, BUR, SNA and ONT. However LAX isn't as large a hub for UA as SFO so you'll see LAX work more often than SFO.