Very good idea to get all the wheels checked over carefully. Straight, true and properly tensioned wheels are goning to be important.
If two of you already have lightweight bikes, pool your money and get the third a real touring bike, then make him carry most of the gear.
Maybe, but I can see that leading to blood shed unless he is a lot stronger than the other two. A slower bike and a heavier load will be a huge handicap in the mountains. In flatter country you could help a bit by letting him draft.
Another approach is to take turns pulling a trailer. I met a father and son who were doing that successfully.