Randomly place 1D string array into 2D char array

I'm trying to randomly place 1D string array into 2D char array but I'm having issues with my for-loop. userWords is 1D array of String while puzzleBoard is a 2D array of char.

I've tried

for(int i=0; i<userWords.length;i++) {
        puzzleBoard[r++] = userWords[i].toCharArray(); 
    }

but it's not placing it randomly like I want it to

So I tried

    for(int i=0; i<userWords.length;i++) {
        int r = rand.nextInt(ROW) + 1;
        int c = rand.nextInt(COLUMN) + 1;
        puzzleBoard[r][c] = userWords[i].charAt(i);
    }

but it's printing only 3 char instead of the 3 strings of char into the char array.

I've also tried

    puzzleBoard[r][c] = userWords[i].toCharArray();

instead of

    puzzleBoard[r][c] = userWords[i].charAt(i);

But it display error "cannot convert from char[] to char"

Thank you

Full Code

public static void main(String[] args) {
    String[] userWords = new String[3];
    Methods.userInput(userWords); //ask user for input 
    Methods.fillPuzzle(puzzleBoard); //fill the puzzle with random char

    for(int i=0; i<userWords.length;i++) {
        int r = rand.nextInt(ROW) + 1;
        int c = rand.nextInt(COLUMN) + 1;
        puzzleBoard[r][c] = userWords[i].charAt(i);
    }

    Methods.printPuzzle(puzzleBoard); //print out the puzzle

}//end main 


public static void printPuzzle(char a[][]) {

    for (int i = 0; i < a.length; i++) {
        for (int j = 0; j < a[i].length; j++) {
            System.out.print(a[i][j] + " ");
        }
        System.out.print((i+1));
        System.out.println();
    }

}//end printPuzzle

public static void fillPuzzle(char a[][]) {

    for (int i = 0; i < a.length; i++) {
        for (int j = 0; j < a[i].length; j++) {
            a[i][j] = '*';
        }
    }
}//end fillPuzzle

public static void userInput(String a[]) {
    Scanner input = new Scanner(System.in);

    for(int i = 0; i < a.length;i++) {
        System.out.println((i+1) + ". enter word:");
        a[i] = input.next().toUpperCase();
    }
}//end userInput