Answer
My study shows that the 3 ribs symbolize 3 provinces or directions into which the Medo-Persian Empire expanded.
Explanation
Through Daniel, God gave the history in advance from Nebuchadnezzar’s Babylonian Empire to Roman Empire that will last till the second coming of the Christ and the establishment of the World Government of God.
The Great Statue with 4 Parts
God enumerates 4 kingdoms that will conquer Jerusalem one after the other.
It begins with Nebuchadnezzar’s Babylonian Empire:
“You O king ………… You are the head of gold” (Dan 2:37-38).
These 4 empires are appearing one after the other. They are not contemporaries. The following verse shows it very clearly:
“And in your place shall arise another kingdom lower than yours,
and another third kingdom of bronze……….” (Dan 2:39).
The fourth and the last empire will be the last because the Kingdom of God will be established in the earth during the fourth empire:
“And in the days of these kings, the God of Heaven shall set up a
kingdom which shall never be destroyed” (Dan 2:44).
[“These kings” are the 10 toes (kings) of the fourth empire as clearly shown in verses 41-43. They are not the 4 world empires.]
So, it is established beyond any doubt that from the time of Daniel till the time of the Second Coming of Christ, there will come only 4 empires that will affect Jerusalem.
4 Beasts Correspond to the 4 Parts of the Great Statue
So, if there are only 4 subsequent world empires that will come and last one by one from Daniel’s time till the end of the age, then we can be sure that the four beasts Daniel describes in chapter 7 are the same empires.
The Lion symbolizes the Babylon, the Bear the Medo-Persian, the Leopard the Greek and the unnamed Beast Rome.
The Bear and the Medo-Persian Empire
Daniel gives more details about this second world empire that will conquer Jerusalem.
The Bear “was raised up on one side” (Dan 7:5).
The same is described in Dan 8:3:
“A ram was standing before the canal, having two horns. And the two
horns were high, but one was higher than the other, and the higher
one came up last”.
The explanation is very clear from history. The Persians came after the Medes and they were more powerful than the latter.
So, the Persian “side” was raised than the Median “side”. The Persian “horn” was higher than the Median “horn”. In short, the Persians came after the Medians and they were more powerful.
The OP Question
The OP question can be answered easily from the following verses:
“and three ribs were in its mouth between its teeth” (Dan 7:5).
As usual, the Scripture explains its own symbols:
“I saw the ram pushing 1) westward and 2) northward and 3) southward” (Dan 8:4).
Conclusion
If one follows the the Scriptural explanations strictly, one understands that the 3 ribs are 3 directions or 3 provinces into which the Medo-Persian Empire expanded.
The Medo-Persians “devoured” (hence “mouth and teeth”) the lands to its West, North and South.