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That's hard to answer without a photo or identity of the trees, the growing conditions and where you are located.

It's doubtful that vole root damage would result in the die-back of lower branches. Root damage usually affects the branch tips and the upper most foliage first.

Shading of the lower branches can cause dieback. Also a pest infestation can affect the lower branches; for example some scale insects like the lower, shaded parts of the plant. Their feeding through sucking mouth parts can cause die-back.

Herbicides applied to the soil surface of a lawn can drift up and affect the lowest branches, as can fire.
Do you have any wildlife that may be feeding on the lower foliage?
You may want to contact your local county extension agent for a definitive diagnosis or a tree arborist.
Here are more ideas:
https://www.gardeningknowhow.com/garden-how-to/info/plant-dropping-leaves.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/what-does-a-dying-tree-look-like-signs-that-a-tree-is-dying.htm

We do not recommend places to purchase. You can try places like Amazon, Ebay, and Etsy. Alternatively, you can try your local gardening centers, or hydroponic shops.

It would be hard to pin down without seeing the tree in question. I can take a few guesses and get you started in the right direction, though.

Usually, this is an indication that it was overwatered. This will lead to an infection which will show as you have described. If the conditions are not wet, or have not been wet, and you are sure that you tree has had a period to dry out thoroughly between waterings then you can rule this out. This article will help you with fungicide use to correct this: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

Pests, such as biting insects can cause brown spotting and leaf problems. This will take a pesticide to correct. These articles will help: https://www.gardeningknowhow.com/special/organic/natural-home-pesticides-organic-garden-pest-control.htm

https://www.gardeningknowhow.com/special/organic/what-are-organic-pesticides.htm

Last, could be a nutrient deficiency. In this case, it depends on how the spots show up, where they are located, and the pH of the soil. A soil test will help you to determine what the soil is lacking in.

This article will help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

It's hard to say when the tree is this far gone. Did you notice anything amiss such as discolored leaves, spots on leaves or stems, leaf wilting? Are they all three the same type of tree or was it different? Do they all have similar cultural requirements? I would consult the local extension agent and discuss the tree's appearance before it died.

https://www.gardeningknowhow.com/extension-search

Trees have a mechanism to sequester and fix problems very similar to humans forming scabs. But it isn't perfect and some trees are better at it than others. An open wound that doesn't heal is an invitation for pests and disease. An attempt should be made to remove the tag. If bark has already grown around it, severe it in several places so it isn't constricting. Once cut, see if the tag can be pulled through. Whether not removing a tag constitutes breach of contract or covered by warranty protection is a legal question I'm not qualified to answer.

Are they planted? If not, go ahead and plant them first and let the roots get established before you prune. Depending on what the plants are, their dormant season would be the best time to cut them back. Generally if they bloom in spring, don't trim till after they flower. If they bloom in summer or fall, it's usually OK to prune in late winter or early spring.
https://www.gardeningknowhow.com/ornamental/shrubs/shgen/rejuvenation-pruning-tips.htm