Tenor: John Denver. Plenty of good material with a wide range to choose from.
?: Randy Travis. I'm not sure where you would classify his voice (he has quite a range) but he is easy to sing along with - very simple orchestrations. He has some quality country cds out, and at least a couple of good gospel cds out (since your folks seem to be gospel related singers)
Bass: Charlie Pride. Again, pretty wide range, but he can really growl on some songs. I love the purity of the tone of his voice. Soft as silk.
I agree with the choice of Anne Murray - very little vibrato which some female singers seem to think is mandatory on every pause in every verse of every song.
They're not today's biggest stars, but do you want quality or today's big names? (ouch, ok - I'll go stick my nose in the corner).