To use an airside lounge in any Heathrow terminal, as far as I know, you need to have a boarding pass that allows you to enter that terminal. If I'm right, and I believe I am, then your relatives won't be able to get into T3 airside to use their day pass.

If your relatives hold passports from a country that doesn't need a transit visa, then they can transfer from T2 to T5 without going through immigration and customs (if they can interline their checked bags), although they will be required to go through security.