The 1837 edition set “glad tiding”. This change was undoubtedly a typo. It was corrected in the 1840 edition and independently in the 1849 LDS edition. Elsewhere in the text, we have only the plural tidings (22 times), of which 11 refer to “glad tidings”. The King James Bible also has only tidings (46 times).
Summary: Maintain the plural usage tidings in 3 Nephi 1:26 and throughout the text.