Q.Lost growth
3-year old Victoria plum tree with only 2 plums. All the leaves are yellow and wilted. Looks very sick.

Plum trees began to bear fruit after they are 3 so your tree is just reaching this age.
There can be a few reasons for the yellowing leaves, including watering issues, insects or disease.
Inspect the plant carefully for insect damage.
Here are some links for you.
https://www.gardeningknowhow.com/edible/fruits/plum/plum-trees-not-fruiting.htm
https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-diseases.htm
https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-pruning.htm