What wire and conduit is required for an attached-garage subpanel?
I love that you're putting a subpanel in your garage.

When you go to buy wire, check the price of aluminum. Last time I did this, I went looking for #8 copper, and got #2 aluminum at 1/2 the price. That was a bundle of 2-2-2-4 SER (service entrance) rated for direct burial, but I put it in conduit anyway.

Your finding that #3 copper will carry 100A looks right to me.

The conduit you want is schedule 40 gray. You'll also need a male adapter at the main panel + a lockwasher + a plastic bushing.

1.4" will allow a max 1" nominal conduit. Big wires are stiff, so bigger conduit is better. You might be able to run one size larger conduit + conduit bodies, and then drop the size just at the garage wall.