Need help. How to create csv file?

Hi I'm a beginner and I have some problem.

I have multiple files in the same directory which has one column but rows following the format.

File: directory/Disk.txt
Content:

a
b
c
d
e

File: directory/Memory.txt

a
b
c
d
e

File: directory/CPU.txt

a
b
c
d
e

I would like to create a report with CSV format and separate each column by "," as below:

a,a,a
b,b,b
c,c,c
d,d,d

e,e,e

Could you please help me to write a shell script. Thank you in advances.

$ paste -d "," CPU.txt  Disk.txt  Memory.txt
a,a,a
b,b,b
c,c,c
d,d,d
e,e,e

try paste..

 
paste -d , <<your filenames>>

It's work! I really appreciated all you guys. You are my HERO !