Agree Bilal, SAPI3 is assigned specifically for SMS text.
LAPDm: the classes associated with handling SMS there are:
Annex F
Six classes and they are Class A to F as set out in e.g.
3GPP TS04.11 v7.1.0 (2000-09) Release 98 or if you want to go back further ETS 300 559: September 1995 (GSM 04.11 version 4.8.0)