Syncing a random variable in ruby

I am making a simple Tic tac toe game where the rows are represented by three arrays and columns are items in those arrays. For my computer choice I use two separate functions to get the row and column.

$first = ["-","-","-"]
$secound = ["-","-","-"]
$third = ["-","-","-"]
def get_row
    row = nil
    case rand(3)
    when 0
        row = $first
    when 1
        row = $secound
    else
        row = $third
    end
    row
end
def get_col
    col = nil
    case rand(3)
    when 0
        col = 0
    when 1
        col = 1
    else 
        col = 2
    end
    col
end

I then use a third function that keeps generating a new "guess" until it finds an empty spot, at which point it marks an 0.

def computer_guess
    temp = false
    try = get_row[get_col]
    while temp == false 
        if try == "-"
           get_row[get_col] = "O"
            temp = true 
        else
            try = get_row[get_col]
        end
    end
end

My problem is that i cant return the guess to the same position that i check for validity. Is the there a way to sync these or do i need a different approach?