RTN11 , your information is incorrect.
The '3 hr' clause you quote (plus other requirements) refers ONLY to FCL.135.A(a), where privileges for a different aeroplane CLASS are sought (e.g. adding TMG privileges to a LAPL(A) with SEP privileges). Whereas FCL.135.A(b) refers to variant or differences training WITHIN a Class and are the same as for any other licence which includes an SEP or TMG Class Rating.