The paper presents the effects of output-coupling power and small signal gain of selenium vapour multiline laser by discharge of either to $^4$He or $^3$He. The purity of the He gas was improved with a special He-fille.. The result shows that compared with those of $^4$He, the output-coupling power and small-signal gain of $^3$He increase in the most of the lines. Especially, the small-signal gain of $^3$He for the strong lines (497.6 nm, 499.3 nm, 506.9 nm, 517.6 nm, 522.8 nm, 530.5 nm) lies about 30% higher than that of $^4$He, and the output-coupling power from doubles to triples.