"Three times the prophet mentions the myrtle trees,could there be some significance in these trees?"
For those of the Single-intent school, a tree is just a tree. However, since literal interpretation permits the use of allegory as allegory, many will use free-for-all allegory to fill in the gaps. This is plain when comparing notes on parables.
But in sensus plenior, everything contributes to speaking of Christ and allegorical/metaphoric meaning is derived from the words themselves and must agree with every other usage.
"Myrtle tree" 01918 הדס hadac is of an uncertain derivation, so we rely on the Hebrew/apostolic practice of deriving the meaning of the word from the embedded metaphor of letters.
echo הד of the fulfilled purpose of God ס
quelled הס by the command of God ד
heard, but not understood ה command ד fulfilled ס
Observe the response of the ones roaming, that all is at rest.
Zec 1:11 And they answered the angel of the LORD that stood among the
myrtle trees, and said, We have walked to and fro through the earth,
and, behold, all the earth sitteth still, and is at rest.
This method is demonstrated many times in the parsing of Hebrew words. "The spirit hovered over the face of the waters" is a verbalization of shmayim "heavens" as it is parsed out. And a verbalization of the letter aleph.
Adam came from the ground Adamah, and was filled with blood 'dam' with the spirit/breath on his face (the aleph). Blood 'dam' is the commandment fulfilled by the son (the cross, the blood).
From other metaphor:
The primary metaphor of tree is the cross. The secondary metaphor, particularly when there are many trees, are those who have been made to be like him (Christ).