Here the typesetter for the 1841 British edition accidentally replaced believest with believeth, an obvious typo which the subsequent LDS edition (1849) corrected. For further discussion of mixups between the archaic inflectional endings -eth and -est, see under 1 Nephi 11:2.
Summary: Maintain in Ether 3:11 the original believest with its inflectional ending -est (“believest thou the words which I shall speak”).