how to generate a key in the below given situation and thus each character pointing to 3 random integer, using this key?

i need to divide a plaintext into a number of blocks (block 1 to block n). Each block represents one row in the plaintext matrix. For each block, an individual key needs to be generated which holds three random integers for each individual character from A to Z in that block and thus each character in the block(with three random integers) is encrypted into one pixel. All ciphered blocks are joined together to form a matrix of pixels.