I think that the web designer and web developper and SEO'er should be in one because of the big and important relation between them. What do you think?

I disagree, these are very specialized fields and you need the best people to carry each one.
I've know very good developers, people who can write excellent C# and VB code, but have no creativity when it comes to design.
They can create a very useful and scalable application that just looks ugly! On the other hand, people who can design a very appealing site with great navigation would know nothing about classes, inheritance, etc.
So if you have the money and care about the product then definitely hire separately, otherwise you have to get the best generalist your money can buy.

for business owners it is very helpfull to find like this person because this will save money and time as they think. this is the unprofessional and (wrong) way to do business but this is what most companies doing

This is the best choice if the person has all that expertese and also know how to get the valuable traffic through Search engine optimization. 

Everyone agrees on the one hand the idea

I agree with this, all in one