Encrypting text using Caeser cipher in CFB mode
Caesar cipher

You are confusing Caesar cipher with block ciphers.

In Caesar cipher the ciphertext is calculated as

$$c_i = p_i + 3 \bmod 26$$

There is no security in Caesar Cipher. A ciphertext only attack possible, as worst scenario.

CFB Caesar cipher

$c_i = c_{i-1} + 3 + p_i \bmod 26 $, with $c_0 = IV$

And, the decryption;

$p_i = c_i - 3 -c_{i-1} \bmod 26 $, with $c_0 = IV$

Example : $P=lucozade = \{11,20,2,14,25,0,3,4\}$

Let choose $IV = f = 5$

\begin{align}
  c_1 &amp;\equiv IV + 3 + p_1 \bmod 26 \equiv 5 + 3 + 11 \bmod 26 \equiv 19 \text { that is } T\\
  c_2 &amp;\equiv c_1 + 3 + p_2 \bmod 26 \equiv 19 + 3 + 20 \bmod 26 \equiv 16 \text { that is } Q\\
  c_3 &amp;\equiv c_2 + 3 + p_3 \bmod 26 \equiv 16 + 3 + 2 \bmod 26 \equiv 19 \text { that is } T\\
  c_4 &amp;\equiv c_3 + 3 + p_4 \bmod 26 \equiv 19 + 3 + 14 \bmod 26 \equiv 10 \text { that is } K\\
\end{align}