I have some family members travelling from Dublin to Faro on BA, via London next year.

They need to transfer from LHR to LGW

So a few questions:
1) Will their bags be checked through all the way from Dublin to Faro?
2) Is the National Express best low cost way to transfer between the airports?
3) Is 3 hours long enough to allow for the transfer between LHR and LGW?