Option 1 states that hydrogen has three isotopes, with protium being the most common. This statement is correct. Hydrogen does indeed have three isotopes - protium, deuterium, and tritium. Protium is the most common and forms about 99.98% of naturally occurring hydrogen.
Option 2 states that the hydrogen ion (H+) exists freely in solution. This statement is incorrect. The hydrogen ion, H+, is a positively charged particle and does exist in solution, but it does not exist freely. It is usually attached to an anion or is bonded with other molecules.
Option 3 states that dihydrogen, H2, acts as a reducing agent. This statement is correct. Dihydrogen gas (H2) can act as a reducing agent as it readily donates its electrons to oxidizing agents, thereby causing reduction in the other substances.
Therefore, the correct answer is option 3: 1 and 3 only. Alert - correct answer should be option 1: 1, 2 and 3.