var array = ["First Cell", "Second Cell", "Third Cell", "Fourth Cell", "Fifth Cell"]
let total = array.count + array.count / 3
for i in 0..<total {
    print("Row index: \(i)")
    if i % 3 == 0 &&  i != 0 {
        print("Add Cell")
    } else {
        let data = array[i]
        print(data)
    }
}

Output:

Row index: 0
First Cell
Row index: 1
Second Cell
Row index: 2
Third Cell
Row index: 3
Add Cell
Row index: 4
Fifth Cell
Row index: 5
Swift/ContiguousArrayBuffer.swift:575: Fatal error: Index out of range //Because, you try to access array[5] which doesn't exists