Yes you need the letters and fearing do the blanks as well.
Don't be tempted to go 001, 002 etc just do 1, 2, 3. Less hassle plus 0's can deform or not take properly and end up looking like 3/ 5/ 6/ 8.
Not a problem if you've only got 5 but a pain if there are 50 of the buggers, we've had to ask the butcher before now as the tattoo becomes easier to read once they have been scaled at the abattoir.