Can you post a link to your editor? If not, I would guess that you need to store the result of the API call into a state, so that it becomes a text string. You might create a state attached to the page itself, and then set the value of the state equal to the the API request you are making in your screenshot. Instead of calling the API directly int the dropdown, try calling that state and you may be able to use “truncate” combined with “merge with” since you are manipulating a page state rather than the API response itself…Bubble is particular in that way sometimes.
Otherwise, you could use the use both regex and “merge with” the results, or figure out a way to combine both of the expressions below into a single regular expression:
Regex for removing first 3 characters: (^[\s\S]{0,3})
Regex for removing last 3 characters: (...)$