Hello liunliun!
Normally I would recommend to have one skeleton, but not a skin per direction if the clothes need to be mixed and matched.
But since you are new to Spine, working with a separate skeleton for each direction may be easier. You can also keep all the directions in the same Spine project but with different skeletons, so that it's easier to use one skeleton animation as the reference for the other directions.
The advantage of having everything in one skeleton is that you can mix and match not only the clothes, but also parts of the body, so that for example the head is looking to the front and the body is looking to the side. The disadvantage is that this setup can be very tricky for beginners and has a longer setup time.
The advantage of having separate skeletons is that you don't have to use complicated procedures to add the different views to the same skeleton, but you just need to rig one view. The disadvantage is that you have 3 times the amount of bones and skins to set, and you can't mix directions.
I suggest to go with the separate skeletons option if you don't foresee to need alternative poses and the 3 directions are enough for you.
By the way, this page features several complete streams with downloadable projects. They are long, but if you are unsure and want to see a complete rigging project, they may be just perfect for you! You can also use the chapters to see only the topics that interest you, skipping the rest of the video. I hope this helps!
Spine: Twitch