Ok so i have seen one post about this and it wasnt that accurrate. Before i give DETAILED instructions on how to do this i want to state that this is NOT a glitch, idk why infinity ward allowed it but i love it. Alright so heres how to do it....
Step 1: open your clan tag and make everything caps (you have to use caps all the time i believe)
Step 2: click on the wierd "A" with this above it "~"
Step 3: after placing the wierd "A", you then place a specific letter to act as the button. In order to have multiple, you do the same thing you do for the first one except with a different letter and adding another wierd "A" followed by the letter of your choice.

These are the letters for each button:
A= a button
B= b button
C= x button
D= y button
E= LB button
F= RB button
J= slides your name down a notch as if someone is below you
M= puts your name inside of your clan tag sorta
N= start button
O= back button or select
P= Left stick
Q= right stick
R= left trigger
S= right trigger

These are the codes for xbox which is the console i play on but i will try to find the letters for PS3 but you get the concept i hope.

Please comment and give me your thoughts guys, it took alot of time to do this and i hope it helps all you hardcore MW3 fans!!!!!!