That’s the problem - the red light in the handle indicates unsafe gear, even though you have three green. You have to have three green AND no light in the handle to know the gear are actually down and locked. You have to cross check both, and if you have either of a red light in the handle and/or no green lights you have to emergency extend the gear and should go straight to that procedure.
…don’t fret - I made the same mistake in a RL trainer. Once…
For JaguarIV and all the other folks less experienced than Stevie: The red light in the handle indicates that the gear are not in the position the handle is in, which is why it illuminates during gear transition. Pretty much what he said, but extended to more situations.