Q.New tiny lemons are falling off my small tree, why?
Bought a 3 gal. tree. It’s planted in the ground.

You can eliminate some of the blossoms or when the tiny lemons have formed, prune some of them away.
Make sure there is no number higher then 8 in your fertilizer that you are using.
Increasing light or adding some additional time with a grow light can help.
Here are some articles with more information.
https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-fruit-drop.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-not-fruiting.htm
https://www.gardeningknowhow.com/edible/fruits/lime/is-lime-fruit-and-lime-blossoms-falling-off-tree-normal.htm
https://www.gardeningknowhow.com/edible/fruits/citrus/growing-citrus-trees-indoors.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/learn-about-fertilizer-for-a-lemon-tree.htm