So a shot is made up of 3 words, and each dragon wall teaches you a word of the shout.

So what happens if you learn the words for a shout out of order? Can you still complete the shot?

Also another question is that some shout guides have certain words assigned to specific walls, but I've learned a word from the same shout when i visited the same walls, does this mean that it doesn't matter which order you visit the walls, as long as you find all 3 walls of the shout?

The words will always be learnt in order, regardless of which wall iirc, as long as you visit all 3 walls of a corresponding shout you'll get all 3 words.

I had one wall teach me all three words of a shout once, unfortunately I don't find that shout useful in any way.