The present invention provides compounds of formula (I) and pharmaceutically acceptable salts thereof, wherein L,X, Ra, Rb, R1, R2 and R3 are as defined in the specification, processes for their preparation, pharmaceutical compositions containing them and their use in therapy.本發明提供式(I)化合物及其醫藥學上可接受之鹽,其中L、X、Ra、Rb、R1、R2及R3係如本說明書中所定義;其製備方法,含有其之醫藥組合物以及其在治療中之用途。