From somebody in comp.sys.cbm, don't remember who nor if I tweaked it further to get this version. The thread was probably “Converting An 8-bit Number Into A String”, but I couldn't find it in Google.
Converts .A to 3 ASCII/PETSCII digits: .Y = hundreds, .X = tens, .A = ones
ldy #$2f ldx #$3a sec - iny sbc #100 bcs - - dex adc #10 bmi - adc #$2f rts