img_size = 20*20*3
x = tf.placeholder(tf.float64, [None, img_size])
W = tf.Variable(tf.zeros([img_size, 3]))
b = tf.Variable(tf.zeros([3]))
y = tf.nn.softmax(tf.matmul(x, W) + b)
y_ = tf.placeholder(tf.float64, [None, 3])