The usage of "Also"

Dear teachers,

I still can't master the usage of "also"

I happened to read the following three sentences with answers from a forum a few days ago by chance. According to what they said: "The meaning of a sentence can be different when the word "also" is written at different position." Though I have a book of Swan Practical English Usage with me (Printed on year 1997, Second edition 1995), non of the explanations from the book under the entries of (also) and ( also, as well as, too) looks similar to what the three sentences and answers described.

I doubt the structure of the sentences and meanings are all correct. Especially sentence 3. I thought "also" cannot be placed at the end of a sentence. Could you kindly explain them in detail? And I want to know the difference between "is also" and "also is".

Here are the three sentences with answers I copied from that forum:
1. William is also concerned in this matter.
= William is concerned in this matter as well as that one.

2. William also is concerned in this matter.
=William is concerned in this matter,just as somebody else is.

3. William is concerned in this matter also.
=This matter is the last of many that have been mentioned.

Your explanation will be very much appreciated!