Before this rotine yo must store the three bytes recived trought serial in CENTENA DECENA UNIDAD, sub the value '0' to every digit to convert the ASCII value to numeric. and:

CONVNUM: MOV B,#100
MOV A,CENTENA
MUL AB
MOV NUMERO,A
MOV B,#10
MOV A,DECENA
MUL AB
ADD A,NUMERO
ADD A,UNIDAD
MOV NUMERO,A
RET
The result is in numero
'como se esperaba'.

Saludos.