Can you assign values to all elements in struct for a 2D array in C?

I want to make a grayscale filter so I have to assign the average of all 3 RGB values to each. I use a 2D array because this is a bitmap image and I'm filtering it by changing each pixel.

This is how the struct is defined (where BYTE is uint8_t):

typedef struct
{
    BYTE  rgbtBlue;
    BYTE  rgbtGreen;
    BYTE  rgbtRed;
} __attribute__((__packed__))
RGBTRIPLE;

Do I need to assign the value to each element of struct separately, like I did below, or is there a way to assign that value to all elements of the struct (since they are the same type) in that specific location in the array at once?

This is the code for the filtering function.

void grayscale(int height, int width, RGBTRIPLE image[height][width])
{
    for (int i = 0; i < height; i++)
    {
        for (int j = 0 ; j < width; j++)
        {
            image[i][j].rgbtBlue = (image[i][j].rgbtBlue + image[i][j].rgbtRed + image[i][j].rgbtGreen)/3;
            image[i][j].rgbtGreen = (image[i][j].rgbtBlue + image[i][j].rgbtRed + image[i][j].rgbtGreen)/3;
            image[i][j].rgbtRed = (image[i][j].rgbtBlue + image[i][j].rgbtRed + image[i][j].rgbtGreen)/3;
        }
    }
    return;
}

You can do it with a compound literal:

RGBTRIPLE x = image[i][j];  // Use temporary to reduce repetition.
BYTE t = (x.rgbtBlue + x.rgbtRed + x.rgbtGreen)/3;
image[i][j] = (RGBTRIPLE) { t, t, t };