Is helium-3 flammable

Answer 1

No.

Helium is an inert gas. It does not react with anything. It can't burn, corrode, etc. anything. That is the main reason they fill airships and balloons with helium.

However, helium is often stored in pressurized tanks. These can explode due to the high pressure of the helium gas inside. As can heluim balloons when they pop. However, this is no more than a tank of compressed air or a normal balloon full of air when it pops.

Most likely you are thinking of hydrogren. This is an explosive gas. When contacted by a flame it combines with oxygen in the air to form water and a lot of chemical energy -- resulting in a large explosion.

Answer 2

No. Helium is a noble gas and is therefore unreactive.