Welcome to the Helpful Gardener!
Do you know your Sunset gardening zone in California? There are some gardening zones in which avocado trees will blossom and set fruit, but the fruit will drop.
For instance, out of 16 listed varieties of avocado in the Sunset Western Garden Book, only 4 will succeed in my local gardening zone. The others will act as yours seems to be acting.
So that's one possibility.
Sunset also states that "pollination [of avocado trees] is complex....Avocado varieties have flowers categorized as either type A or type B, depending on the time of day they open and when pollen is released. For best production, combine a type A with a type B variety; flower type for each varieiy is noted [on the opposite page]."
So maybe your tree needs a "friend" of the opposite flower type.
Varieties of apricot also differ in their chill, water, and heat requirements. Pruning is different for them than for many other trees, as is winter dormant treatment. Again from Sunset: "Apricot trees bear most of their fruit on short spurs that form on the previous year's growth and remain fruitful for about 4 years....essential treatment dates are during dormancy, before and after flowering, and at red-bud stage."
Sunset then lists over two dozen (!) varieties of apricot, but again, only four are recommended for my particular gardening zone.
Since you're experiencing difficulty with lemon, avocado, *and* apricot, could there be some lack of summer heat to mature these fruits? Or perhaps the specific varieties play a part as well.
Knowing more about your climate zone, especially the Sunset zone, would help a lot.
You can usually find a reference copy of the Western Garden Book (covering climate and gardening conditions for all states and provinces west of the Continental Divide) at independent garden-supply stores/nurseries. The maps are excellent.
(Or just send me a PM--personal/private message with your city and county, and I'll look on the maps for you.)
USDA Zone 9, Sunset Zone 17
This is all just a starting point.