(1) According to the comparative experiment of large-particle potatoes and small-particle potatoes, the smaller the particles, the faster the reaction speed, so carrots can promote the decomposition of hydrogen peroxide faster. The method that can be adopted is to cut carrots into smaller particles;

(2) Because the air in the conical flask was pressed into the jar at the beginning of the reaction, and the jar contained a lot of air, the wooden strips with fire were put into the jar, but it was found that the wooden strips could not be re-ignited.

(3) Assuming that the solute in 34g 15% H2O2 solution is completely decomposed, the oxygen mass of X can be obtained:

2H2O2？ Catalyst? . ? 2H2O+O2↑

68 32

34g× × 15% x

According to: 6832 = 34g× 15% x and X=2.4g, the volume of oxygen is 2.4g1.43/l =1.678l. 。

So the answer is: (1) Cut carrots into smaller particles; (2) The purity of oxygen is not high enough; (3) 1.678 liters.