Our apple tree wasn't doing much of anything (it barely even ever flowered), so I finally cut it down and bark grafted 3 different varieties onto it - Liberty, Ashmead, and Cox Orange Pippin. These should all be able to pollinate one another. We also have a Honeycrisp tree nearby that should benefit from this, too.
There are a lot of YouTube videos showing various types of grafts. I would look there first - I learned from watching videos. I also reference this a good bit: https://www.wikihow.com/Graft-a-Tree (but you can't beat watching a video of someone do it).
For my tree I did what's called a bark graft, which is often used for trees that stop producing or to change the variety. There are also bud grafts, whip and tongue grafts, cleft grafts, etc.
Be sure also to get varieties that are good for your growing zone and that flower around the same time so they can pollinate one another.
Super neat! We have a Japanese Cherry Willow, which is a graft. We inherited it with the house and are slowly trying to bring it back to better health. It got pretty unbalanced, but it's already looking better each year. Grafting is such an interesting thing!
Yes it should. I picked varieties that bloom around the same time, so they should pollinate one another. It's like having 3 separate trees, they're all just connected to the same root stock.
Check out the whip and tongue graft. Very similar idea to what you described but it's much more secure and I want to say more of the cambium layer is joined together.
To add to what wizzor said, it can depend a lot on the variety of root stock you're grafting onto. Some root stocks naturally produce dwarfed trees, which can be nice if you don't have a lot of space. Even if that isn't the case, though, you can just prune it to a height you like.