At some point I shared the following microflow on modelshare: This should give you an idea of how to build the name:
The trim() functions you see in the expression are to remove excess white spaces from the name for example if somebody would write the text 'Anthony ' in the firstname you don't want to have the fullname say 'Anthony Korah' instead of 'Anthony Korah' (that extra white space in the name looks kind of weird.
The following help page goes over a couple of options in more detail: https://world.mendix.com/display/public/howto50/String+Concatenation
https://modelshare.mendix.com/models/60fd9671-0e98-4ee3-9719-e0a060abe844/bcocontactpersoncreatefullname?embed=true