2 Removed backticks and typo.
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You could use commas for the first level, and em-dashes for the second level. Since the second level is composed solely of verbs and the third level is composed of nouns again, you can phrase it like so:

The park has bears, deer — which are friendly, like to eat popcorn, marshmallows, and chips, and like to watch visitors — and other animals — which live in tall trees and short trees.

It is important to note that the verbs must all be in the same structure at the point where they begin. You can't say "are friendly, like to eat ..., and watch..." You have to say "are friendly, like to eat..., and like to watch..." The first way would be suitable for "like to eat..., watch..., and make friends..."

Whenever you have 3 or more structures in a list, especially when they include a verb, you have to make sure they all break at the same point. Eg: if the order of words were dictated by numbers, you could have "1a 2a 3a, 2b 3b, and 2c 3c." But you could not have "1a 2a 3a, 2b 3b, and 3c." If sentence part 1 does not make sense with sentence part 3 (eg "go ... the store") then it becomes unclear if you are implying 2a or 2b between 1a and 3c.

Semicolons should be reserved for bringing two complete sentences closer together; they are like periods where the sentences are related. This is sentence is incorrect; when this clause can't stand alone.

This sentence is incorrect; when this clause can't stand alone.

You could use commas for the first level, and em-dashes for the second level. Since the second level is composed solely of verbs and the third level is composed of nouns again, you can phrase it like so:

The park has bears, deer — which are friendly, like to eat popcorn, marshmallows, and chips, and like to watch visitors — and other animals — which live in tall trees and short trees.

It is important to note that the verbs must all be in the same structure at the point where they begin. You can't say "are friendly, like to eat ..., and watch..." You have to say "are friendly, like to eat..., and like to watch..." The first way would be suitable for "like to eat..., watch..., and make friends..."

Whenever you have 3 or more structures in a list, especially when they include a verb, you have to make sure they all break at the same point. Eg: if the order of words were dictated by numbers, you could have "1a 2a 3a, 2b 3b, and 2c 3c." But you could not have "1a 2a 3a, 2b 3b, and 3c." If sentence part 1 does not make sense with sentence part 3 (eg "go ... the store") then it becomes unclear if you are implying 2a or 2b between 1a and 3c.

Semicolons should be reserved for bringing two complete sentences closer together; they are like periods where the sentences are related. This is sentence is incorrect; when this clause can't stand alone.

You could use commas for the first level, and em-dashes for the second level. Since the second level is composed solely of verbs and the third level is composed of nouns again, you can phrase it like so:

The park has bears, deer — which are friendly, like to eat popcorn, marshmallows, and chips, and like to watch visitors — and other animals — which live in tall trees and short trees.

It is important to note that the verbs must all be in the same structure at the point where they begin. You can't say "are friendly, like to eat ..., and watch..." You have to say "are friendly, like to eat..., and like to watch..." The first way would be suitable for "like to eat..., watch..., and make friends..."

Whenever you have 3 or more structures in a list, especially when they include a verb, you have to make sure they all break at the same point. Eg: if the order of words were dictated by numbers, you could have "1a 2a 3a, 2b 3b, and 2c 3c." But you could not have "1a 2a 3a, 2b 3b, and 3c." If sentence part 1 does not make sense with sentence part 3 (eg "go ... the store") then it becomes unclear if you are implying 2a or 2b between 1a and 3c.

Semicolons should be reserved for bringing two complete sentences closer together; they are like periods where the sentences are related.

This sentence is incorrect; when this clause can't stand alone.

1
source | link

You could use commas for the first level, and em-dashes for the second level. Since the second level is composed solely of verbs and the third level is composed of nouns again, you can phrase it like so:

The park has bears, deer — which are friendly, like to eat popcorn, marshmallows, and chips, and like to watch visitors — and other animals — which live in tall trees and short trees.

It is important to note that the verbs must all be in the same structure at the point where they begin. You can't say "are friendly, like to eat ..., and watch..." You have to say "are friendly, like to eat..., and like to watch..." The first way would be suitable for "like to eat..., watch..., and make friends..."

Whenever you have 3 or more structures in a list, especially when they include a verb, you have to make sure they all break at the same point. Eg: if the order of words were dictated by numbers, you could have "1a 2a 3a, 2b 3b, and 2c 3c." But you could not have "1a 2a 3a, 2b 3b, and 3c." If sentence part 1 does not make sense with sentence part 3 (eg "go ... the store") then it becomes unclear if you are implying 2a or 2b between 1a and 3c.

Semicolons should be reserved for bringing two complete sentences closer together; they are like periods where the sentences are related. This is sentence is incorrect; when this clause can't stand alone.