If foliage yellow u may be overwatering. But it is normal for tree to drop fruit. As the mature they will drop less

The foliage is nice and green with lots of new growth. I guess the tree is just too young then :(

try to water them at least 3 times a week, there's mulch on top

That might be too much, and too often. A thorough soaking once every week or two should be enough, especially with the mulch. Get a moisture meter and use it, not a calendar, to know how much water to apply.

Citrus set far more fruit than they can mature, and shed the excess, so it might also be normal fruit drop.

if its in a pot and out in full sun, watering 3 times a week will be ok. but if its in the ground with mulch, then that'll be too much water.

like others have said, citrus will bloom profusely, then drop fruit throughout the summer.

They are in plastic containers. How big do they need to get before they stop dropping the fruit?