@Tony Chatfield1 updated the post and an image is added

Based on details provided a before update BR like this should work.

var myArray = ['NA', 'NA', 'NA'];

if (u_case_id_1 != '') {
myArray[0] = u_case_id_1;
}
if (u_case_id_2 != '') {
myArray[1] = u_case_id_2;
}
if (u_case_id_3 != '') {
myArray[2] = u_case_id_3;
}

current.u_connect_case_ids = myArray.toString();

//Note edited syntax error

@Anna_Servicenow 

why not use List type field ?

Hi @Anna_Servicenow ,

This is how you can achieve this. write business rule or client script per your requirement.

var caseId1 = 111;
var caseId2 = 200;
var caseId3 = 120;

// Concatenate the values with a comma separator
var caseIdArr = [caseId1, caseId2, caseId3].filter(Boolean).join(', ');

gs.print(caseIdArr);