Can you put 1 male betta with two neon tetras in a 1.5 gallon tank?

1.5 Gallon? That is a small tank even for a betta and two neon tetras. A typical rule of thumb is 1 inch of fish per gallon of water, so you should only have 1.5 inch of fish. A betta is 2 inches or more, so the neons would be asking for trouble. Both type of fish are great, so my advice would be to get a bigger tank and put them together. Neons are schooling fish and would like to be in groups of three or more.