I am thinking on try Joomla 3 to make my dynamic web sites, instead of doing them from scratch. I need to make a program that receives users client records and meting agenda. Then the user may be able to see the client list by filtering some fields. Can I make that in Joomla by modules or will I have to modify php files too much?
Of course that in some points I will have to do some code. Or is it many?
Install Joomla and have a look at the backend (for example contacts or banners components). If the functionality you need looks in principle similar (editing records, CRUD methods), have a look at tutorials on building custom components.
As a side-note, of course it's better to build websites / web applications using a framework, whatever you like, so that you don't have to write so much code and get benefits like security and others.
I think by "modules" you mean "extensions" in Joomla terminology. WIthout knowing more, I'd say you might want a forms component like Fabrik or similar (visit extensions.joomla.org).